c++ - Why is my specialized template function invoked only in debug builds? -

i have templated functions in c++11 xcode project , of them have specializations. however, have discovered specializations called in debug builds; if build in release, ignored.

i have created simple example:

special.h

#include <cstdio>  struct special {     template<typename t>     void call(const t&) { puts("not special"); } }; 

special.cpp

#include "special.h" #include <string>  template<> void special::call(const std::string&) { puts("very special"); } 

main.cpp

#include "special.h" #include <string>  int main() {     std::string str = "hello world";     special s;     s.call(123);     s.call(str); } 

you can download project (until somewhere summer of 2013 @ least) reproduce issue if don't want create yourself. first run project debug configuration, run again in release. output expect is:

not special
special

and indeed debug build configuration. however, release, this:

not special
not special

which means specialized implementation of special::call in special.cpp ignored.

why result inconsistent? should ensure specialized function called in release builds?

your program has ub. explicit specialisation or @ least declaration must visible before being used. [temp.expl.spec]§6:

if template, member template or member of class template explicitly specialized specialization shall declared before first use of specialization cause implicit instantiation take place, in every translation unit in such use occurs; no diagnostic required.

add declaration special.h:

template<> void special::call(const std::string&); 

alternatively, can put specialistation header. however, specialisation no longer template, follows normal function rules , must marked inline if placed in header.

also, careful function template specialisations have rather specific behaviour, , it's better use overloads specialisations. see herb sutter's article details.