Если так, то почему? Почему он не использует конструктор копирования типа значения?

Я получаю следующую ошибку:

/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/vector.tcc: In member functio
n `ClassWithoutAss& ClassWithoutAss::operator=(const ClassWithoutAss&)':
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/vector.tcc:238:   instantiate
d from `void std::vector<_Tp, _Alloc>::_M_insert_aux(__gnu_cxx::__normal_iterato
r<typename _Alloc::pointer, std::vector<_Tp, _Alloc> >, const _Tp&) [with _Tp =
ClassWithoutAss, _Alloc = std::allocator<ClassWithoutAss>]'
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_vector.h:564:   instantia
ted from `void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = Class
WithoutAss, _Alloc = std::allocator<ClassWithoutAss>]'
main.cpp:13:   instantiated from here
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/vector.tcc:238: error: non-st
atic const member `const int ClassWithoutAss::mem', can't use default assignment
 operator

запуск g ++ main.cpp для следующего кода:

/*
 * ClassWithoutAss.h
 *
 */

#ifndef CLASSWITHOUTASS_H_
#define CLASSWITHOUTASS_H_

class ClassWithoutAss
{

public:
    const int mem;
    ClassWithoutAss(int mem):mem(mem){}
    ClassWithoutAss(const ClassWithoutAss& tobeCopied):mem(tobeCopied.mem){}
    ~ClassWithoutAss(){}

};

#endif /* CLASSWITHOUTASS_H_ */

/*
 * main.cpp
 *
 */

#include "ClassWithoutAss.h"
#include <vector>

int main()
{
    std::vector<ClassWithoutAss> vec;
    ClassWithoutAss classWithoutAss(1);
    (vec.push_back)(classWithoutAss);

    return 0;
}