Odd behaviour of g++ and clang++ with enabled optimization [duplicate] - c++

Question

This question already has answers here:
Why full specialization of template function is not picked up from the .cpp file without declaration?
(3 answers)
Closed 4 years ago.
Here's my program:
print.hpp:
#pragma once
#include <iostream>
template<size_t p>
void print()
{
std::cout << "" << __FILE__ << "" << __LINE__ << "" << std::endl;
exit(0);
}
print.cpp:
#include "print.hpp"
template<>
void print<13>()
{
std::cout << "Unlucky." << std::endl;
}
main.cpp:
#include <iostream>
#include "print.hpp"
int main()
{
std::cout << "Started." << std::endl;
print<13>();
std::cout << "Exiting." << std::endl;
}
When I compile that with g++ main.cpp print.cpp -O0 -std=c++11 && ./a.out it works fine (output is:
Started.
Unlucky.
Exiting.
).
However, if'd I compile that with g++ main.cpp print.cpp -O1 -std=c++11 && ./a.out it would give me a segmentation fault with the output:
Started.
Unlucky.
Speicherzugriffsfehler //German for memory access error
Almost the same with clang++, without optimization it would do its job just fine
and with -O1 or higher it outputs that:
Started.
Unlucky.
./print.hpp8
Why is that?

Answer 1

You need to declare the template specialization in the .hpp file.
template<size_t p>
void print()
{
std::cout << "" << __FILE__ << "" << __LINE__ << "" << std::endl;
exit(0);
}
// Declare the specialization.
template<> void print<13>();
Without the declaration in the .hpp file, I get a linker error with g++ 6.4.0.
.../Local/Temp/cctCC5MK.o:print.cc:(.text+0x0): multiple definition of `void print<13ul>()'
.../Local/Temp/ccgodRUG.o:socc.cc:(.text$_Z5printILm13EEvv[_Z5printILm13EEvv]+0x0): first defined here
collect2: error: ld returned 1 exit status
I am not sure how you are able to successfully build your program without the declaration.