output

g++ --std=c++0x -c ui2.cpp
g++ --std=c++0x  test.cpp ui2.o
/tmp/cc4DASkN.o: In function `main':
test.cpp:(.text+0x20): undefined reference to `std::function<bool (int const&)> ui2::check_true<int>()'
collect2: ld gab 1 als Ende-Status zurüc

test.cpp

#include "ui2.h"

int main(){
        std::cout << ui2::quert(4) << "\n" //works
	          << ui2::check_true<int>()(4) << "\n"; //does not link!
}

ui2.cpp

#include "ui2.h"

int ui2::quert(int bar){return bar;};

template <class T> std::function<bool(const T&)> ui2::check_true(){
	return [](const T& m) { return true; };
}

ui2.h

#pragma once

#include <functional>

namespace ui2{
	int quert(int bar);	
	
	template <class T> std::function<bool(const T&)> check_true();
};