This is my first time asking a question, so apologies if it is not done 100%:
I have a class which saves and loads a binary file with a specific data structure.
If the program creates an instance of the class, save the binary file, and creates another instance of the class to load/read the binary file consequently, everything seems 100% correct.
However, if I run the program to save the binary file and then run it again to load/read that binary file, it gives me a segmentation fault at the end.
The program still does everything it needs to do before the segmentation fault, except deconstructing the class at the end (obviously).
It looks like my allocation of the memory is not correct, but I am not sure where I am going wrong.
A simplified version of the code follow (also here: https://github.com/LenteDreyer/Tests.git )
Can someone see where I am going wrong?
class header file that save/loads the file
#ifndef __TESTS_MAP_HH__
#define __TESTS_MAP_HH__
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>
#include <sstream>
typedef struct test_struct{
bool test_bool;
double test_double;
std::vector<double> test_vector;
} test_struct_t;
class map
{
private:
std::string m_path, m_str;
double m_double;
test_struct m_struct;
public:
map(const std::string& a_id);
void set_str(std::string a_str);
void set_double(double a_double);
void set_struct(test_struct a_struct);
void load_file();
void save_file() const;
void show_file() const;
~map();
};
#endif //__TESTS_MAP_HH__
class source file that save/loads the binary file
#include "map.hh"
map::map(const std::string& a_id)
{
m_path = a_id + ".bin";
m_str = "none";
m_double = 0.0;
m_struct = {false, 0.0};
}
void map::set_str(std::string a_str){
m_str = a_str;
}
void map::set_double(double a_double){
m_double = a_double;
}
void map::set_struct(test_struct a_struct){
m_struct = a_struct;
}
void map::load_file(){
std::ifstream l_inF;
l_inF.open(m_path.c_str(), std::ios::binary | std::ios::in);
l_inF.read((char*)&m_double,sizeof(double));
l_inF.read((char*)&m_struct,sizeof(test_struct_t));
size_t str_size;
l_inF.read((char*)&str_size, sizeof(str_size));
m_str.resize(str_size);
l_inF.read((char*)&m_str[0], str_size);
l_inF.close();
}
void map::save_file() const{
std::ofstream l_outF;
l_outF.open(m_path.c_str(), std::ios::binary | std::ios::out);
l_outF.write((char*)&m_double,sizeof(double));
l_outF.write((char*)&m_struct,sizeof(test_struct_t));
size_t str_size = m_str.size();
l_outF.write((char*)&str_size, sizeof(str_size));
l_outF.write((char*)&m_str[0], str_size);
l_outF.close();
}
void map::show_file() const{
std::cout << ">>-----------------------------------------------" << std::endl;
std::cout << ">> double : " << m_double << std::endl;
std::cout << ">> double : " << m_double << std::endl;
std::cout << ">> struct.bool : " << m_struct.test_bool << std::endl;
std::cout << ">> struct.double : " << m_struct.test_double << std::endl;
std::cout << ">> struct.vector : " << "size = " << m_struct.test_vector.size() << std::endl;
std::cout << ">> string : " << m_str << std::endl;
std::cout << ">>-----------------------------------------------" << std::endl;
}
map::~map(){}
main function case 1 works, and case 2 gives the segmentation fault.
#include "map.hh"
int main(int argc, char const *argv[])
{
std::string id = "mapfile";
int action = 0;
if(argc > 1) action = std::stoi(argv[1]);
else {
std::string input;
std::cout << "Enter case (1 or 2): ";
std::cin >> input;
action = std::stoi(input);
}
switch (action)
{
case 1:
{
// This works 100% (no errors and it saves/reads class perfectly)
std::vector<double> l_vect = {0.1, 0.0, 0.6};
test_struct save_struct = {true, 5.0, l_vect};
map test_save(id);
test_save.show_file();
test_save.set_double(8.0);
test_save.set_str("save this string");
test_save.set_struct(save_struct);
test_save.show_file();
test_save.save_file();
map test_load(id);
test_load.load_file();
test_load.show_file();
}
break;
case 2:
{
// gives segmentation error at the end of the program
map test_load(id);
test_load.load_file();
test_load.show_file();
}
break;
default:
break;
}
return 0;
}
I'm trying to use boost library 1.55.0 to create a shared memory.I have a example.cpp file which creates boost shared memory. Mexing of this file is successful,but throws the following exception while debugging "MATLAB.exe has triggered a breakpoint".Is this exception because of the version of the boost being incompatible with the matlab version? How to resolve this
`/* File : sfun_counter_cpp.cpp
* Abstract:
*
* Example of an C++ S-function which stores an C++ object in
* the pointers vector PWork.
*
* Copyright 1990-2000 The MathWorks, Inc.
*
*/
#include "iostream"
#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/containers/vector.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
typedef struct
{
int outGate;
unsigned int outPin;
int inGate;
unsigned int inPin;
} wire;
typedef struct
{
unsigned int gateType;
unsigned int inPins;
unsigned int outPins;
std::vector<wire> inWires;
std::vector<wire> outWires;
} gate;
std::vector<gate> gates;
wire wiredata;
gate gatedata;
class counter {
double x;
public:
counter() {
x = 0.0;
}
double output(void) {
x = x + 1.0;
return x;
}
};
#ifdef __cplusplus
extern "C" { // use the C fcn-call standard for all functions
#endif // defined within this scope
#define S_FUNCTION_LEVEL 2
#define S_FUNCTION_NAME sfun_counter_cpp
/*
* Need to include simstruc.h for the definition of the SimStruct and
* its associated macro definitions.
*/
#include "simstruc.h"
/*====================*
* S-function methods *
*====================*/
/* Function: mdlInitializeSizes ===============================================
* Abstract:
* The sizes information is used by Simulink to determine the S-function
* block's characteristics (number of inputs, outputs, states, etc.).
*/
static void mdlInitializeSizes(SimStruct *S)
{
/* See sfuntmpl_doc.c for more details on the macros below */
ssSetNumSFcnParams(S, 1); /* Number of expected parameters */
if (ssGetNumSFcnParams(S) != ssGetSFcnParamsCount(S)) {
/* Return if number of expected != number of actual parameters */
return;
}
ssSetNumContStates(S, 0);
ssSetNumDiscStates(S, 0);
if (!ssSetNumInputPorts(S, 0)) return;
if (!ssSetNumOutputPorts(S, 1)) return;
ssSetOutputPortWidth(S, 0, 1);
ssSetNumSampleTimes(S, 1);
ssSetNumRWork(S, 0);
ssSetNumIWork(S, 0);
ssSetNumPWork(S, 1); // reserve element in the pointers vector
ssSetNumModes(S, 0); // to store a C++ object
ssSetNumNonsampledZCs(S, 0);
ssSetOptions(S, 0);
}
/* Function: mdlInitializeSampleTimes
=========================================
* Abstract:
* This function is used to specify the sample time(s) for your
* S-function. You must register the same number of sample times as
* specified in ssSetNumSampleTimes.
*/
static void mdlInitializeSampleTimes(SimStruct *S)
{
ssSetSampleTime(S, 0, mxGetScalar(ssGetSFcnParam(S, 0)));
ssSetOffsetTime(S, 0, 0.0);
}
#define MDL_START /* Change to #undef to remove function */
#if defined(MDL_START)
/* Function: mdlStart
=======================================================
* Abstract:
* This function is called once at start of model execution. If you
* have states that should be initialized once, this is the place
* to do it.
*/
static void mdlStart(SimStruct *S)
{
ssGetPWork(S)[0] = (void *) new counter; // store new C++ object in
the
} // pointers vector
#endif /* MDL_START */
/* Function: mdlOutputs =======================================================
* Abstract:
* In this function, you compute the outputs of your S-function
* block. Generally outputs are placed in the output vector, ssGetY(S).
*/
static void mdlOutputs(SimStruct *S, int_T tid)
{
using namespace boost::interprocess;
counter *c = (counter *) ssGetPWork(S)[0]; // retrieve C++ object from
shared_memory_object::remove("MySharedMemory");
//create the shared memory
managed_shared_memory segment(create_only, "MySharedMemory", 65536);
//create the allocators for the struct elements to be accessed as
vectors
typedef allocator<gate,
managed_shared_memory::segment_manager>gate_alloc;
typedef allocator<wire,
managed_shared_memory::segment_manager>inwire_alloc;
typedef allocator<wire,
managed_shared_memory::segment_manager>outwire_alloc;
//create a boost vector with an associated allocator to it
typedef vector<gate, gate_alloc>gate_vec;
typedef vector<wire, inwire_alloc>inwire_vec;
typedef vector<wire, outwire_alloc>outwire_vec;
//Initialize shared memory STL-compatible allocator
const gate_alloc alloc_inst(segment.get_segment_manager());
const inwire_alloc alloc_inst1(segment.get_segment_manager());
const outwire_alloc alloc_inst2(segment.get_segment_manager());
//construct the segment for pushing the data into it
gate_vec *gate_data = segment.construct<gate_vec>("gatedata")
(alloc_inst);
inwire_vec *inwire_data = segment.construct<inwire_vec>("inwiredata")
(alloc_inst1);
outwire_vec *outwire_data = segment.construct<outwire_vec>
("outwiredata")
(alloc_inst2);
//push the data into the vectors
wiredata.inGate = 10;
wiredata.inPin = 2;
wiredata.outGate = 1;
wiredata.outPin = 3;
inwire_data->push_back(wiredata);
outwire_data->push_back(wiredata);
gatedata.gateType = 1;
gatedata.inPins = 2;
gatedata.outPins = 3;
gate_data->push_back(gatedata);
real_T *y = ssGetOutputPortRealSignal(S,0); // the pointers vector
and use
y[0] = c->output(); // member functions of
the
} // object
/* Function: mdlTerminate
=====================================================
* Abstract:
* In this function, you should perform any actions that are necessary
* at the termination of a simulation. For example, if memory was
* allocated in mdlStart, this is the place to free it.
*/
static void mdlTerminate(SimStruct *S)
{
counter *c = (counter *) ssGetPWork(S)[0]; // retrieve and destroy C++
delete c; // object in the
termination
} // function
/*======================================================*
* See sfuntmpl_doc.c for the optional S-function methods *
*======================================================*/
/*=============================*
* Required S-function trailer *
*=============================*/
#ifdef MATLAB_MEX_FILE /* Is this file being compiled as a MEX-file?*/
#include "simulink.c" /* MEX-file interface mechanism */
#else
#include "cg_sfun.h" /* Code generation registration function */
#endif
#ifdef __cplusplus
} // end of extern "C" scope
#endif
`
this is the sfunction which I have to mex , debug and run .Though the mexing of the above code snippet is successful ,it throws an exception "MATLAB.exe has triggered a breakpoint" while debugging.
I can give you an example. Unfortunately I cannot test it with windows, but I have tested it on a UNIX system. The main idea is the same. In this case it is shared memory from an external binary to a Matlab mex function.
The external binary is:
#include <cstring>
#include <string>
#include <cstdlib>
#include <iostream>
#include <boost/interprocess/shared_memory_object.hpp>
#include <boost/interprocess/mapped_region.hpp>
using namespace boost::interprocess;
const std::string payload("SHARED MEMORY CONTENT");
int main(void) {
shared_memory_object shm(open_or_create, "memory4mat" ,read_write);
shm.truncate(payload.size());
mapped_region mem(shm, read_write);
std::memcpy(mem.get_address(), payload.c_str(), mem.get_size());
do {
std::cout << '\n' << "Press a key to continue...";
} while (std::cin.get() != '\n');
shared_memory_object::remove("memory4mat");
return 0;
}
while the mex function is:
#include "mex.hpp"
#include "mexAdapter.hpp"
#include "MatlabDataArray.hpp"
#include <string>
#include <cstdlib>
#include "boost/interprocess/shared_memory_object.hpp"
#include "boost/interprocess/mapped_region.hpp"
using namespace boost::interprocess;
class MexFunction : public matlab::mex::Function {
public:
void operator()(matlab::mex::ArgumentList outputs, matlab::mex::ArgumentList inputs) {
matlab::data::ArrayFactory factory;
shared_memory_object shm(open_only, "memory4mat", read_only);
mapped_region mem(shm, read_only);
std::string payload(static_cast<const char *>(mem.get_address()), mem.get_size());
outputs[0] = factory.createCharArray(payload);
outputs[1] = factory.createScalar<int16_t>(mem.get_size());
}
};
it uses the C++ Interface and Data API for Matlab. To compile the two example you need to add the boost include directory as compiler options (shared memory is a header only feature in boost).
The external binary create a shared memory that contains the string "SHARED MEMORY CONTENT", and waits an enter from the user to remove the shared memory object.
The mex files opens the shared memory if exist (if the shared memory do not exist an error is reported and handled in Matlab, which is one of the reasons why I prefer C++ api) and copy its content in a Matlab char array. The function returns two values, the first is the content of the shared memory, the second is the length of the shared memory (the mapper uses all the memory, set with truncate).
This simple example uses only basic features and should work on Unix and Windows system, but again I cannot test on win.
A more complete example
Let's try with a more complete example about shared memories and Matlab Mex files. Let us write a very simple external binary that allows us to create/delete/read/write a shared memory. This binary has a lot of stuff hardcoded for simplicity, such as the name of the memory file ("shmem"):
// File: share_server.cpp
// g++ share_server.cpp -o share_server
#include <cstring>
#include <string>
#include <cstdlib>
#include <iostream>
#include <boost/interprocess/shared_memory_object.hpp>
#include <boost/interprocess/mapped_region.hpp>
using namespace boost::interprocess;
static const std::size_t size = 20;
static const std::size_t wsize = 15;
static const char name[6] = "shmem";
static const char input[wsize] = "write in share";
char output[size];
inline void printHelp() {
std::cout << "Options:" << std::endl;
std::cout << " n) open a new 'shmem' memory" << std::endl;
std::cout << " d) delete a 'shmem' memory" << std::endl;
std::cout << " r) read from 'shmem' memory" << std::endl;
std::cout << " w) write to 'shmem' memory" << std::endl;
std::cout << " x) Exit" << std::endl;
}
inline void cmd_createShare() {
try {
shared_memory_object sm(create_only, name, read_write);
sm.truncate(size);
std::cout << "Shared object created" << std::endl;
} catch(std::exception & e) {
std::cout << "Create Error :: " << e.what() << std::endl;
}
}
inline void cmd_deleteShare() {
try {
shared_memory_object::remove(name);
std::cout << "Shared object deletetd" << std::endl;
} catch(std::exception & e) {
std::cout << "Delete Error:: " << e.what() << std::endl;
}
}
inline void cmd_readShare() {
try {
shared_memory_object sm(open_only, name, read_only);
mapped_region sh_mem(sm, read_only);
std::string ret(static_cast<const char *>(sh_mem.get_address()), sh_mem.get_size());
std::cout << ret << std::endl;
} catch(std::exception & e) {
std::cout << "Read Error:: " << e.what() << std::endl;
}
}
inline void cmd_writeShare() {
try {
shared_memory_object sm(open_only, name, read_write);
mapped_region sh_mem(sm, read_write);
std::memcpy(sh_mem.get_address(), input, wsize);
std::cout << "Write completed" << std::endl;
} catch(std::exception & e) {
std::cout << "Read Error:: " << e.what() << std::endl;
}
}
we can write 3 mex files (using the C++ api) in order to interact with the shared memory. The first one, the simplest, reads the content of the shared memory as a string and returns it to the Matlab workspace. The interface in Matlab syntax is something like:
function [value, read_size] = read_share(share_name)
...
end
and the C++ implementation is the following:
// File: read_share.cpp
#include "mex.hpp"
#include "mexAdapter.hpp"
#include "MatlabDataArray.hpp"
#include <string>
#include <cstring>
#include <cstdlib>
#include <vector>
#include <exception>
#include "boost/interprocess/shared_memory_object.hpp"
#include "boost/interprocess/mapped_region.hpp"
using namespace boost::interprocess;
using namespace matlab::data;
class MexFunction : public matlab::mex::Function {
private:
std::shared_ptr<matlab::engine::MATLABEngine> engine;
ArrayFactory factory;
void throwError(std::string errorMessage) {
engine->feval(matlab::engine::convertUTF8StringToUTF16String("error"),
0, std::vector<Array>({ factory.createScalar(errorMessage) }));
}
uint64_t read_shared_memory(const std::string & name, std::string & ret_value) {
try {
shared_memory_object sm(open_only, name.c_str(), read_only);
mapped_region sh_mem(sm, read_only);
ret_value += std::string(static_cast<const char *>(sh_mem.get_address()), sh_mem.get_size());
return ret_value.size();
} catch(std::exception & e) {
throwError(std::string("Reading error: ") + std::string(e.what()));
}
return 0;
}
void checkArguments(matlab::mex::ArgumentList inputs, matlab::mex::ArgumentList outputs) {
if (inputs.size() != 1)
throwError("Input must be of size 1");
if (inputs[0].getType() != ArrayType::CHAR)
throwError("First element must be a matlab char array");
if (outputs.size() > 2)
throwError("Too many outputs (required 1)");
}
public:
MexFunction() {
engine = getEngine();
}
void operator()(matlab::mex::ArgumentList outputs, matlab::mex::ArgumentList inputs) {
checkArguments(inputs, outputs);
const CharArray name_array = std::move(inputs[0]);
std::string name = name_array.toAscii();
std::string ret_string("");
uint64_t ret_size = read_shared_memory(name, ret_string);
outputs[0] = factory.createScalar(ret_string);
outputs[1] = factory.createScalar<uint64_t>(ret_size);
}
};
The second mex file is the write operation. It takes two input: the name of the shared memory and the string to write inside the memory. The mex checks the maximum size of the shared memory and stores no more than the available space. The function returns the bytes written in the The interface for the write function is something like:
function written_size = write_share(share_name, string)
...
end
and the implementation is:
// File: write_share.cpp
#include "mex.hpp"
#include "mexAdapter.hpp"
#include "MatlabDataArray.hpp"
#include <string>
#include <algorithm>
#include <cstring>
#include <cstdlib>
#include <exception>
#include "boost/interprocess/shared_memory_object.hpp"
#include "boost/interprocess/mapped_region.hpp"
using namespace boost::interprocess;
using namespace matlab::data;
class MexFunction : public matlab::mex::Function {
private:
std::shared_ptr<matlab::engine::MATLABEngine> engine;
ArrayFactory factory;
void throwError(std::string errorMessage) {
engine->feval(matlab::engine::convertUTF8StringToUTF16String("error"),
0, std::vector<Array>({ factory.createScalar(errorMessage) }));
}
uint64_t write_shared_memory(const std::string & name, const std::string & value) {
try {
shared_memory_object sm(open_only, name.c_str(), read_write);
mapped_region sh_mem(sm, read_write);
uint64_t size = std::min(value.size(), sh_mem.get_size());
std::memcpy(sh_mem.get_address(), value.c_str(), size);
return size;
} catch(std::exception & e) {
throwError(std::string("Reading error: ") + std::string(e.what()));
}
return 0;
}
void checkArguments(matlab::mex::ArgumentList inputs, matlab::mex::ArgumentList outputs) {
if (inputs.size() != 2)
throwError("Input must be of size 2");
if (inputs[0].getType() != ArrayType::CHAR)
throwError("First element must be a matlab char array");
if (inputs[1].getType() != ArrayType::CHAR)
throwError("Second element must be a matlab char array to save");
if (outputs.size() > 1)
throwError("Too many outputs (required 1)");
}
public:
MexFunction() {
engine = getEngine();
}
void operator()(matlab::mex::ArgumentList outputs, matlab::mex::ArgumentList inputs) {
checkArguments(inputs, outputs);
const CharArray name_array = std::move(inputs[0]);
std::string name = name_array.toAscii();
const CharArray value_array = std::move(inputs[1]);
std::string value = value_array.toAscii();
uint64_t written = write_shared_memory(name, value);
outputs[0] = factory.createScalar<uint64_t>(written);
}
};
The last mex is the most complex and handles the creation and deletion of the shared memory. You will notice the presence of a destructor that handles the removal of shared memory when the mex is unloaded from Matlab. The interface takes a command in the form of "create" or "delete", a string with the name of the share and the size of the shared memory for creation (it must be an unsigned int - uint16(...)). The function returns the size of the shared memory (it should be equal to size):
function size_shmem = menage_mex(command, share_name, uint16(size))
...
end
the implementation is the following:
// File: menage_share.cpp
#include "mex.hpp"
#include "mexAdapter.hpp"
#include "MatlabDataArray.hpp"
#include <string>
#include <cstring>
#include <cstdlib>
#include <vector>
#include <exception>
#include "boost/interprocess/shared_memory_object.hpp"
#include "boost/interprocess/mapped_region.hpp"
using namespace boost::interprocess;
using namespace matlab::data;
class MexFunction : public matlab::mex::Function {
private:
std::shared_ptr<matlab::engine::MATLABEngine> engine;
ArrayFactory factory;
std::vector<std::string> pool;
void throwError(std::string errorMessage) {
engine->feval(matlab::engine::convertUTF8StringToUTF16String("error"),
0, std::vector<Array>({ factory.createScalar(errorMessage) }));
}
uint64_t run_command(const std::string & cmd, const std::string & name, uint64_t size) {
if (cmd == "create")
return create_shared_memory(name, size);
if (cmd == "delete")
return delete_shared_memory(name, size);
throwError("The command is unknown");
return 0;
}
uint64_t create_shared_memory(const std::string & name, uint64_t size) {
bool in_pool = false;
for (const auto & el : pool) {
if (el == name) {
in_pool = true;
break;
}
}
if (in_pool) {
try {
shared_memory_object sm(open_only, name.c_str(), read_only);
mapped_region sm_reg(sm, read_only);
if (sm_reg.get_size() != size)
throwError("Memory already exist and it is of different size");
return 0;
} catch (std::exception & e) {
throwError(std::string("Cannot open existing shared memory (maybe already open?) :: ") + std::string(e.what()));
}
} else {
try {
shared_memory_object sm(create_only, name.c_str(), read_write);
sm.truncate(size);
pool.push_back(name);
return size;
} catch (std::exception & e) {
throwError(std::string("Cannot create shared memory [" + name + "] (maybe already open?) :: ") + std::string(e.what()));
}
}
return 0;
}
uint64_t delete_shared_memory(const std::string & name, uint64_t size) {
std::size_t in_pool = 0;
for (const auto & el : pool) {
if (el == name)
break;
in_pool++;
}
if (in_pool < pool.size()) {
shared_memory_object::remove(name.c_str());
pool.erase(pool.begin() + in_pool);
} else {
throwError("Shared memory [" + name + "] is not handled by this mex");
}
return 0;
}
void checkArguments(matlab::mex::ArgumentList inputs, matlab::mex::ArgumentList outputs) {
if (inputs.size() != 3)
throwError("Input must be of size 3");
if (inputs[0].getType() != ArrayType::CHAR)
throwError("First element must be a matlab char array");
if (inputs[1].getType() != ArrayType::CHAR)
throwError("Second element must be amatlab char array");
if (inputs[2].getType() != ArrayType::UINT64)
throwError("Third element must be a single uint64 integer");
if (outputs.size() > 1)
throwError("Too many outputs (required 1)");
}
void inputArguments(std::string & cmd, std::string & name, uint64_t & size, matlab::mex::ArgumentList inputs) {
const CharArray cmd_array = std::move(inputs[0]);
const CharArray name_array = std::move(inputs[1]);
const TypedArray<uint64_t> size_array = std::move(inputs[2]);
cmd = cmd_array.toAscii();
name = name_array.toAscii();
size = size_array[0];
}
public:
MexFunction() {
pool.clear();
engine = getEngine();
}
~MexFunction() {
for (const auto & el : pool) {
shared_memory_object::remove(el.c_str());
}
}
void operator()(matlab::mex::ArgumentList outputs, matlab::mex::ArgumentList inputs) {
checkArguments(inputs, outputs);
std::string cmd, name;
uint64_t size;
inputArguments(cmd, name, size, inputs);
uint64_t ret = run_command(cmd, name, size);
outputs[0] = factory.createScalar<uint64_t>(ret);
}
};
To compile the mex you can use the following script:
MEX_OPT = ['-I', '/path/to/boost'];
MEX_SRC = { ...
'menage_share.cpp', ...
'read_share.cpp', ...
'write_share.cpp' ...
};
for i = 1:length(MEX_SRC)
mex(MEX_OPT, MEX_SRC{i});
end
!g++ share_server.cpp -o share_server
and you can test them as follows:
(MATLAB) | (TERMINAL)
>> menage_share('create', 'shmem', uint64(20)) |
<< 20 |
>> write_share('shmem', 'Hello there') | $ ./share_server
<< 11 | ( ... help message ... )
| << r
| >> Hello there
I've recently read that if you use an object of a class as a reciving parameter of a function, a copy of an object has to be created automaticly. Therefore, if the destructor is included in the class, both original object and it's copy will be vanished automaticly. However, when I tried to make a small code with the same conception destructor only activated once. What can cause the problem? Thanks in advance!
#include "stdafx.h"
#include <iostream>
using namespace std;
class MyClass {
int val;
public:
MyClass(int i)
{
val = i;
cout << "Constructor is in progress" << endl;
}
void SetVal(int i)
{
val = i;
}
int GetVal()
{
return val;
}
~MyClass()
{
cout << "Destructer is in progress" << endl;
}
};
void Display(MyClass obj)
{
cout << obj.GetVal();
}
int main()
{
MyClass a(10);
cout << "Before display()" << endl;
Display(a);
cout << "After display()" << endl;
system("pause");
return 0;
}
It is called after the return statement. The first message you are seeing is from the copied object. When you get to system("pause") your original object is still in scope, so the destructor is not called. It is called after the return statement is evaluated.
Is destructor called at the end of main(); strange behavior
The following program aims to instantiate and use the singleton pattern class proposed by Loki Astari and accepted as answer at the following link.
C++ Singleton design pattern
Note the addition of a simple counter, by way of the private counter variable, along with the increment() mutator, and getCtr() accessor methods.
Expected program output is:
0
1
Press any key to exit...
The actual output is
0
0
Press any key to exit...
Why is the counter in the singleton class not being incremented as expected?
What follows is a minimal, complete, and verifiable program, written to illustrate the issue.
#include "stdafx.h"
#include <iostream>
#include <string>
class S {
public:
static S & getInstance() {
static S instance;
instance.counter = 0; // initialize counter to 0
return instance;
}
S(S const &) = delete;
void operator = (S const &) = delete;
void increment() { ++counter; }
int getCtr() { return counter; }
private:
S() {}
int counter;
};
int main() {
S * s; // s is a pointer to the singleton object
S * t; // t is another pointer to the singleton object.
std::cout << s->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
std::cout << "Press any key to exit...";
std::cin.get();
return 0;
}
Thx, Keith :^)
your problem is you are initializing the counter inside the
getInstance() method
instead, initialize it inside the constructor
your code should be like the following,
#include <iostream>
#include <string>
class S {
public:
static S & getInstance() {
static S instance;
// instance.counter = 0; // initialize counter to 0
return instance;
}
S(S const &) = delete;
void operator = (S const &) = delete;
void increment() { ++counter; }
int getCtr() { return counter; }
private:
S() {counter =0;}
int counter;
};
int main() {
S * s; // s is a pointer to the singleton object
S * t; // t is another pointer to the singleton object.
std::cout << s->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
std::cout << "Press any key to exit...";
std::cin.get();
return 0;
}
then the Output will be
0
1
2
3
4
5
Press any key to exit...
Usually, callBack() gotta be overridden in Child class.
but it doesn't. When the thread calls callBack(), it runs original method.
Is there any way to right this?
I compiled it with "g++ -o file source.cpp -lpthread"
I'm sure it is not about a compiler.
#include <iostream>
#include <unistd.h>
#include <pthread.h>
using namespace std;
class Parent
{
public:
virtual void callBack()
{
cout << "Original callBack() reported this: " << this << endl;
}
private:
pthread_t th = 0;
static void *th_func(void *arg)
{
Parent *p = (Parent*)arg;
cout << "*th_func() reported *arg: " << arg << endl;
p->callBack();
}
public:
Parent()
{
if(pthread_create(&th, NULL, th_func, (void*)this) < 0)
cerr << "thread not born." << endl;
else
cout << "thread has born." << endl;
}
~Parent()
{
if(th!=0)
pthread_join(th, NULL);
cout << "joined. Parent leaving." << endl;
}
};
class Child : public Parent
{
public:
void callBack()
{
cout << "child overridden." << endl;
}
Child() : Parent(){}
};
int main()
{
Child *ch = new Child();
delete ch;
return 0;
}
The problem with your code is that you are calling the thread function from inside the parent constructor. At that point, the Child object is not constructed yet (look up object initialisation order in C++), thus the only virtual function that it can call is the parent's.
From a C++ point of view, it's doing the right thing :).
In order to get your code to work, you have to separate the thread creation from the object creation, otherwise you'll never be able to call a function in a derived class.
Here's some more info from the C++ FAQ. And here's what Scott Meyers has to say about this topic.