Weird uninitialized const member behavior

Question

Consider this code snippet:

struct Foo {

};

template<typename T>
struct Bar {
    const T foo;
};

int main() {
    Bar<Foo> test;
}

I'm compiling it with g++-4.9.2 with [-std=c++11 -O0 -g3 -pedantic -Wall -Wextra -Wconversion] and getting error: uninitialized const member in ‘struct Bar<Foo>’. This is pretty obvious.

BUT try just add std::string as Foo member and program compiles!

#include <string>
struct Foo {
    std::string test;
};
// (...)

What happening? Replacing test's type into double causes program to fail to compile again. What string member changes in class?

Link to online compiler with this snippet.

It seems that gcc behaves like that since version 4.6.

Answer 1

I think it should consistently always produce an error. Clang does so. The C++ standard says in clause (4.3) of §12.1.4 that the default constructor is implicitly deleted when

— any non-variant non-static data member of const-qualified type (or array thereof) with no brace-or equal-initializer does not have a user-provided default constructor,

As Foo does not have a user-provided default constructor, Bar should have an implicitly deleted default constructor and thus instantiating Bar<Foo> test in main should produce an error.

Maybe report a bug to GCC?

Answer 2

If you have const data member in your class/struct, then compiler won't generate default constructor for that. You have to explicitly define it and initialize that const member ( not assign it ).

It should be an error in both cases.