"function template has already been defined" using "std::enable_if_t"

Question

I am trying to use std::enable_if_t to switch implementations of a template function based on a trait of the type passed to it.

Here is an example:

#include <iostream>
#include <type_traits>

// Enable if "T" is integral
template <typename T,
    typename = std::enable_if_t<std::is_integral_v<T>>
>
void print(T value)
{
    std::cout << "Integral: " << value << std::endl;
}

// Enable if "T" is not integral
template <typename T,
    typename = std::enable_if_t<!std::is_integral_v<T>>
>
void print(T value)
{
    std::cout << "Not Integral: " << value << std::endl;
}

int main()
{
    int i = 42;
    print(i);

    double d = 42.0;
    print(d);
}

The issue is, this fails to compile stating:

'void print(T)': function template has already been defined.

This feels odd to me since std::is_integral_v<T> and !std::is_integral_v<T> will never both evaluate to true at the same time so whenever one implementation is enabled, the other should be disabled.

Why does this not work? And what is the best way to resolve this and get the functionality I am looking for?

Answer 1

CPP Reference:

A common mistake is to declare two function templates that differ only in their default template arguments. This does not work because the declarations are treated as redeclarations of the same function template (default template arguments are not accounted for in function template equivalence).

/*** WRONG ***/

struct T {
    enum { int_t,float_t } m_type;
    template <typename Integer,
              typename = std::enable_if_t<std::is_integral<Integer>::value>
    >
    T(Integer) : m_type(int_t) {}

    template <typename Floating,
              typename = std::enable_if_t<std::is_floating_point<Floating>::value>
    >
    T(Floating) : m_type(float_t) {} // error: treated as redefinition
};

/* RIGHT */

struct T {
    enum { int_t,float_t } m_type;
    template <typename Integer,
              std::enable_if_t<std::is_integral<Integer>::value, int> = 0
    >
    T(Integer) : m_type(int_t) {}

    template <typename Floating,
              std::enable_if_t<std::is_floating_point<Floating>::value, int> = 0
    >
    T(Floating) : m_type(float_t) {} // OK
};

Care should be taken when using enable_if in the type of a template non-type parameter of a namespace-scope function template. Some ABI specifications like the Itanium ABI do not include the instantiation-dependent portions of non-type template parameters in the mangling, meaning that specializations of two distinct function templates might end up with the same mangled name and be erroneously linked together. For example:

// first translation unit

struct X {
    enum { value1 = true, value2 = true };
};

template<class T, std::enable_if_t<T::value1, int> = 0>
void func() {} // #1

template void func<X>(); // #2

// second translation unit

struct X {
    enum { value1 = true, value2 = true };
};

template<class T, std::enable_if_t<T::value2, int> = 0>
void func() {} // #3

template void func<X>(); //#4

The function templates #1 and #3 have different signatures and are distinct templates. Nonetheless, #2 and #4, despite being instantiations of different function templates, have the same mangled name in the Itanium C++ ABI (_Z4funcI1XLi0EEvv), meaning that the linker will erroneously consider them to be the same entity.

Fix:

#include <iostream>
#include <type_traits>

// Enable if "T" is integral
template <typename T,
    std::enable_if_t<std::is_integral_v<T>, int> = 0
>
void print(T value)
{
    std::cout << "Integral: " << value << std::endl;
}

// Enable if "T" is not integral
template <typename T,
    std::enable_if_t<!std::is_integral_v<T>, int> = 0
>
void print(T value)
{
    std::cout << "Not Integral: " << value << std::endl;
}

int main()
{
    int i = 42;
    print(i);

    double d = 42.0;
    print(d);
}

---

Better yet, use concepts:

#include <iostream>
#include <concepts>

// Enable if "T" is integral
template <std::integral T>
void print(T value)
{
    std::cout << "Integral: " << value << std::endl;
}

// Enable if "T" is not integral
template <typename T>
void print(T value)
{
    std::cout << "Not Integral: " << value << std::endl;
}

int main()
{
    int i = 42;
    print(i);

    double d = 42.0;
    print(d);
}

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