CODEWITHSUNDEEP
c++templatesextern
Khuldraeseth na'Barya
Khuldraeseth na'Barya

Reputation: 730

Explicit specialization of variable template

How can I manage explicit specialization of a variable template?

I have in a header:

// foo.h
#pragma once
template<typename T> extern T minBound;

And in a single nearby compilation unit:

// foo.cpp
#include "foo.h"
template<> int minBound<int> = 0x80000000;
template<> short minBound<short> = 0x8000;

And a main:

// main.cpp
#include <iostream>
#include "foo.h"

int main() {
    std::cout << minBound<int> << std::endl; // Hopefully -2147483648
    std::cout << minBound<short> << std::endl; // Hopefully -32768
    return 0;
}

Compiled with g++ *.cpp.

The linker tells me that I have multiple definition of minBound<int> and multiple definition of minBound<short>. Can variable templates not be extern? What I have in mind is different values for various template specializations; how might I go about accomplishing this?

I'm on Ubuntu 18.04.1, gcc version 7.4.0. Tested it on WSL using GCC 7.4 and 8.3; no issue.

I can just make it a zero-argument function, I know, but that's boring.

Upvotes: 2

Views: 246

Answers (1)

Davis Herring
Davis Herring

Reputation: 40063

Any explicit specialization is like a normal variable or function in that it must be declared everywhere it’s used (i.e., in a header) and defined in one source file. For a variable template, the non-defining declaration contains extern, just like for any other variable. However, GCC doesn’t seem to support this (per Wandbox).

Upvotes: 1

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