I have a template class with a specialization, defined in another file. Therefore it is possible to generate two versions of the same class: one time by replacing the template parameter and one time using the specialization. My current understanding is, that this can lead to two instanciations of the same type to have different sizes in memory, resulting in segmentation faults.
I created a minimal example and the following code is to illustrate the question:
Create a template class:
// - templateexample.h ---------------
#ifndef TEMPLATEEXAMPLE_H
#define TEMPLATEEXAMPLE_H
template<typename T> class Example
{
public:
Example(){}
int doWork() {return 42;}
};
#endif
// -----------------------------------
Template specialization in another file:
// - templatespecialization.h --------
#ifndef TEMPLATESPECIALIZATION_H
#define TEMPLATESPECIALIZATION_H
#include "templateexample.h"
template<> class Example<int>
{
public:
Example() : a(0), b(1), c(2), d(3) {}
int doWork() {return a+b+c+d;}
private:
int a; //<== the specialized object will be larger in memory
int b;
int c;
int d;
};
#endif
// --------------------------------
Have a class which includes only the template class definition, but should include the specialization.
// - a.h --------------------------
#ifndef A_H
#define A_H
#include "templateexample.h"
class A
{
public:
Example<int> returnSmallExample();
};
#endif
// - a.cpp ------------------------
#include "a.h"
Example<int> A::returnSmallExample() {return Example<int>();}
// --------------------------------
The main class now knows two versions of Example<int> the one from A and the one from the templatespecialization.h.
// - main.cpp ---------------------
#include <iostream>
#include "a.h"
#include "templatespecialization.h"
int main()
{
A a;
Example<int> test = a.returnSmallExample();
std::cout<<test.doWork()<<std::endl;
}
// --------------------------------
Please note, this problem will only occur when compiling class A separately, this example from ideone outputs 6, whereas using separate files can result in a segmentation fauls, or output 42 (https://ideone.com/3RTzlC). On my machine the example compiles successfully and outputs 2013265920:
In the production version of the above example everything is build into a shared library which is used by main.
Question 1: Why doesn't the linker detect this problem? This should be easy to spot by comparing the size of objects.
Question 2: is there a way to examine the object files or the shared library to detect multiple implementations of the same type like in the example above?
Edit: please note: the code above is a minimal example to explain the problem. The reason for the situation is that the template class is from one library and I cannot edit the files from this library. Finally the whole thing is used all over the place in the executable and now I need to find out if the problem above occurs.
Edit: code above can be compiled like this:
#!/bin/bash
g++ -g -c a.cpp
g++ -g -c main.cpp
g++ -o test a.o main.o
