What would be the reason that the explicit instantiation failed? - c++

Question

/* MyClass.h */
class MyClass
{
public:
template <typename T>
void Foo(const T &val);
};
/* MyClass.cpp */
#include <string>
#include <iostream>
#define EXPLICIT_INSTANTIATION_FOO(MyType) \
template void MyClass::Foo(const MyType &val)
EXPLICIT_INSTANTIATION_FOO(int);
EXPLICIT_INSTANTIATION_FOO(float);
EXPLICIT_INSTANTIATION_FOO(std::string);
template <typename T>
void MyClass::Foo(const T &val)
{
std::cout << "My Value: " << val << std::endl;
}
For some other reason, I couldn't define the Foo in the header file, then I try to explicitly instantiate all the types I need in the source file. And it works pretty well on g++-4.8.
Then once it I submit the codes, I find the building failed on Windows 10, x86_64, cl19. It was an unresolved symbol error there and was look like my instantiations weren't be exposed from the object.
But interestingly, when I try to reproduce this error on my own Windows by making that simple snippet above. I found it works well on my vs2017 with cl19...
There would be thousands of configuration differs between the company and the personal machine, but I just don't have any idea that what would be the reason why the instantiations don't work there... Any ideas appreciated! :)
SUPPLEMENTS:
FYI, the complete codes are here:
/* MyClass.h */
#pragma once
class MyClass
{
public:
MyClass();
~MyClass();
template <typename T>
void Foo(const T &val);
};
/* MyClass.cpp */
#include "MyClass.h"
#include <iostream>
#include <string>
#define EXPLICIT_INSTANTIATION_FOO(MyType) \
template void MyClass::Foo(const MyType &val)
EXPLICIT_INSTANTIATION_FOO(int);
EXPLICIT_INSTANTIATION_FOO(float);
EXPLICIT_INSTANTIATION_FOO(std::string);
MyClass::MyClass() {}
MyClass::~MyClass() {}
template <typename T>
void MyClass::Foo(const T &val)
{
std::cout << "My Value: " << val << std::endl;
}
/* main.cpp */
#include "MyClass.h"
#include <iostream>
#include <string>
int main()
{
std::string v = "hello";
MyClass cls;
cls.Foo(v);
std::cin.get();
return 0;
}
Which passed the compilation on msvc.

Answer 1

Move the function definition to before the EXPLICIT_INSTANTIATION_FOO invocations. This is something that never should have worked but compilersllowed, they are slowly coming around on it. If you want to make msvc reject it use the /permissive- flag.

Answer 2

Thanks, everybody, this silly question is solved.
The reason is the template instantiations will not inherit the dllexport attribute from the template definition on Windows. And obviously, it works on my silly test codes because it wasn't compiled as the DLL :)