How to link nested templated specializations? - c++

Question

I would like to be able to use a generic string parsing function as follows:
Utils::parse::fromString<int>(whatever)
Utils::parse::fromString<float>(whatever)
Following the suggestion here, I moved template specialization to separate *.ipp files, which are #includeed from the *.hpp:
Utils.hpp
#ifndef UTILS_HPP_
#define UTILS_HPP_
#include <string>
namespace Utils
{
namespace parse
{
template<typename T>
T fromString(std::string s);
}
}
#include "Utils.ipp"
#endif // UTILS_HPP_
Utils.ipp
#ifndef UTILS_IPP_
#define UTILS_IPP_
template<>
int Utils::parse::fromString<int>(std::string s) { return 42; }
template<>
float Utils::parse::fromString<float>(std::string s) { return 42.0; }
#endif // UTILS_IPP_
Now I have a class with a specialized method:
Foo.hpp
#ifndef FOO_HPP_
#define FOO_HPP_
#include <string>
#include "Utils.hpp"
class Foo
{
public:
Foo();
template<typename T>
T get(std::string s);
};
#include "Foo.ipp"
#endif // FOO_HPP_
Foo.ipp
#ifndef FOO_IPP_
#define FOO_IPP_
#include <string>
template<typename T>
T Foo::get(std::string s)
{
return Utils::parse::fromString<T>(s);
}
#endif // FOO_HPP_
test.cpp
#include <iostream>
#include <string>
#include "Utils.hpp"
#include "Foo.hpp"
// Custom template specialization
template<>
char Utils::parse::fromString<char>(std::string s)
{
return 'c';
}
int main()
{
// Calling `fromString` directly - this works!
const std::string whatever = "whatever";
std::cout << Utils::parse::fromString<int>(whatever) << std::endl;
std::cout << Utils::parse::fromString<char>(whatever) << std::endl;
// Calling `fromString` via `Foo` - linking error!
Foo foo;
std::cout << foo.get<int>(whatever) << std::endl;
return 0;
}
If I only use fromString directly, it works fine. However, if I use Foo, i get "multiple definition" linking errors:
g++ test.cpp Foo.cpp
/tmp/cc7ACcVe.o: In function `int Utils::parse::fromString<int>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)':
Foo.cpp:(.text+0x0): multiple definition of `int Utils::parse::fromString<int>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)'
/tmp/ccrN171X.o:test.cpp:(.text+0x0): first defined here
/tmp/cc7ACcVe.o: In function `float Utils::parse::fromString<float>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)':
Foo.cpp:(.text+0xf): multiple definition of `float Utils::parse::fromString<float>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)'
/tmp/ccrN171X.o:test.cpp:(.text+0xf): first defined here
collect2: error: ld returned 1 exit status

Answer 1

Whereas template functions doesn't require inline to avoid multiple definitions, specialization does as regular functions.
You have to add it:
template <>
inline int Utils::parse::fromString<int>(std::string s) { return 42; }
template<>
inline float Utils::parse::fromString<float>(std::string s) { return 42.0; }