template specialization causes a undefined reference error

Question

I tried a template specialization like bellow.

#include<iostream>
using namespace std;

template<class T1, class T2>
T1 something(T2 a);

template<class T2>
double something(T2 a){
    double b;
    return b;
}

int main(){
    something<double, double>(0.0);
}

However, this gives me a compilatoin error:

In function `main':
test.cpp:(.text+0x9): undefined reference to `double something<double, double>(double)'

Could you tell me how to fix it?

I can compile this exact code without error - are you sure this is **exactly** the code you are compiling? How are you compiling it? — BoBTFish, Sep 07 '18 at 06:03
@BoBTFish I am really sorry, I pasted different one. Now, I edited. — orematasaburo, Sep 07 '18 at 06:06
This is **not** specialization, it is actually **overloading**: https://stackoverflow.com/a/8061522/1171191 — BoBTFish, Sep 07 '18 at 06:21
You are getting that error because you are calling a function (overload) that is declared but not defined. — Daniel Langr, Sep 07 '18 at 06:45
Possible duplicate of [What is an undefined reference/unresolved external symbol error and how do I fix it?](https://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix) — Daniel Langr, Sep 07 '18 at 06:48

songyuanyao · Accepted Answer · 2018-09-07T06:28:50.063

This is not template specialization, but function template overloading.

The 1st overloading has two template parameters, the 2nd one has only one; when you call it with two specified template arguments like something<double, double>(0.0);, the 1st one will be selected in overload resolution; but it's not defined then leads to link error.

^{BTW: Function templates can only be full specialized, they can't be partial specialized. And in most cases function template overloading would do the work well.}

template specialization causes a undefined reference error

1 Answers1