11.10 — Using function templates in multiple files

Consider the following program, which doesn’t work correctly:

main.cpp:

#include <iostream>

template <typename T>
T addOne(T x); // function template forward declaration

int main()
{
    std::cout << addOne(1) << '\n';
    std::cout << addOne(2.3) << '\n';

    return 0;
}

add.cpp:

template <typename T>
T addOne(T x) // function template definition
{
    return x + 1;
}

If addOne were a non-template function, this program would work fine: In main.cpp, the compiler would be satisfied with the forward declaration of addOne, and the linker would connect the call to addOne() in main.cpp to the function definition in add.cpp.

But because addOne is a template, this program doesn’t work, and we get a linker error:

1>Project6.obj : error LNK2019: unresolved external symbol "int __cdecl addOne<int>(int)" (??$addOne@H@@YAHH@Z) referenced in function _main
1>Project6.obj : error LNK2019: unresolved external symbol "double __cdecl addOne<double>(double)" (??$addOne@N@@YANN@Z) referenced in function _main

In main.cpp, we call addOne<int> and addOne<double>. However, since the compiler can’t see the definition for function template addOne, it can’t instantiate those functions inside main.cpp. It does see the forward declaration for addOne though, and will assume those functions exist elsewhere and will be linked in later.

When the compiler goes to compile add.cpp, it will see the definition for function template addOne. However, there are no uses of this template in add.cpp, so the compiler will not instantiate anything. The end result is that the linker is unable to connect the calls to addOne<int> and addOne<double> in main.cpp to the actual functions, because those functions were never instantiated.

The most conventional way to address this issue is to put all your template code in a header (.h) file instead of a source (.cpp) file:

add.h:

#ifndef ADD_H
#define ADD_H

template <typename T>
T addOne(T x) // function template definition
{
    return x + 1;
}

#endif

main.cpp:

#include "add.h" // import the function template definition
#include <iostream>

int main()
{
    std::cout << addOne(1) << '\n';
    std::cout << addOne(2.3) << '\n';

    return 0;
}

That way, any files that need access to the template can #include the relevant header, and the template definition will be copied by the preprocessor into the source file. The compiler will then be able to instantiate any functions that are needed.

You may be wondering why this doesn’t cause a violation of the one-definition rule (ODR). The ODR says that types, templates, inline functions, and inline variables are allowed to have identical definitions in different files. So there is no problem if the template definition is copied into multiple files (as long as each definition is identical).

But what about the instantiated functions themselves? If a function is instantiated in multiple files, how does that not cause a violation of the ODR? The answer is that functions implicitly instantiated from templates are implicitly inline. And as you know, inline functions can be defined in multiple files, so long as the definition is identical in each.

Here’s another example of a function template being placed in a header file, so it can be included into multiple source files:

max.h:

#ifndef MAX_H
#define MAX_H

template <typename T>
T max(T x, T y)
{
    return (x < y) ? y : x;
}

#endif

foo.cpp:

#include "max.h" // import template definition for max<T>(T, T)
#include <iostream>

void foo()
{
	std::cout << max(3, 2) << '\n';
}

main.cpp:

#include "max.h" // import template definition for max<T>(T, T)
#include <iostream>

void foo(); // forward declaration for function foo

int main()
{
    std::cout << max(3, 5) << '\n';
    foo();

    return 0;
}

In the above example, both main.cpp and foo.cpp #include "max.h" so the code in both files can make use of the max<T>(T, T) function template.