Question

Using gcc version 4.4.3 as follows:

gcc -g -x c++ -lstdc++ -std=c++98 -o ./main ./main.cpp

this code in main.cpp compiles fine:

#include <iostream>

struct A
{
    A()
    : m_flag(false)
    {
    }

    const bool m_flag;
};

static A aa = A();

int main(int argc, char* argv[])
{
    A a;

    // Not static = copy OK
    A b( a );
    A c = b;
    A d = A();

    // Static = copy not OK
    // aa = A();
}

But if I uncomment aa = A(); I get:

./main.cpp: In member function 'A& A::operator=(const A&)':
./main.cpp:4: error: non-static const member 'const bool A::m_flag', can't use default assignment operator
./main.cpp: In function 'int main(int, char**)':
./main.cpp:24: note: synthesized method 'A& A::operator=(const A&)' first required here 

Why does the default copy construction and copy assignment work for copies on the stack but not when replacing the non-const static with a copy?

Answer 1

The problem is this line:

 const bool m_flag;
 ...
 aa = A();  // invokes 'A::operator =(const A&)`

which invokes default operator =. Modifying const member is a trivial error.

A b( a );   // all invoke `A::A(const A&)`
A c = b;  
A d = A(); 

invokes default copy constructor (not operator = as you assume), where m_flag is assigned a new value at the initialization itself.