Undefined reference to const int inside shared_ptr

Question

I have a Config class

// config.hpp class Config { public: static constexpr int a = 1; static constexpr int b = 1; } 

and include in main.cpp

 // main.cpp #include "config.hpp" int main () { std::cout << Config::a << std::endl; // this is ok std::shared_ptr<otherClass> stream = std::make_shared<otherClass>( Config::a); // compile error } 

and the compiler said undefined reference to Config::a

and it works when using cout , but does not work in the shared_ptr constructor.

I have no idea why this is happening.

Answer 1

Note that std :: make_shared takes a parameter by reference, which leads to Config::a odr-used , since it will be bound to the reference parameter, then it needs to be defined in the namespace area (before C ++ 17).

On the other hand, std::cout << Config::a will not use Config::a to use odr, because std :: basic_ostream :: operator <<<<<<<<<<<<<<<<<< <; (int) takes a parameter by value, Config::a then undergoes the lvalue-to-r transformation requested to copy-initialize the parameter, so Config::a not used odr.

If an object is used by odr, its definition must exist. You can add a definition (to the implementation file) as

 constexpr int Config::a; // only necessary before C++17 

Note that it cannot have an initializer.

Live

Since the C ++ 17 constexpr static data element is implicitly embedded, then such a definition is not required again, so your code works well from C ++ 17.

If the static data member is declared constexpr , it is implicitly inline and does not need to be updated in the namespace scope. This update without an initializer (previously required as shown above) is still allowed, but is deprecated.

Live

Answer 2

Your a is private and either public: it needs to execute it or make the struct class accessible by default. But this compiles under C ++ 14 https://godbolt.org/g/tS4M1Z

 #include <iostream> #include <memory> struct Config { static constexpr int a = 1; static constexpr int b = 1; }; struct otherClass { otherClass( int c ) { } }; int main () { std::cout << Config::a << std::endl; // this is ok std::shared_ptr<otherClass> stream = std::make_shared<otherClass>( Config::a ); // compile error }