Does std::vector use the assignment operator of its value type to push_back elements?

Question

If so, why? Why doesn't it use the copy constructor of the value type?

I get the following error:

/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

running g++ main.cpp on the following code:

/*
 * 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;
}

Answer 1

The C++03 standard says elements must be copy-constructible and copy-assignable to be used in a standard container. So an implementation is free to use whichever it wants.

In C++0x, these requirements are put on a per-operation basis. (In general, elements must be move-constructible and move-assignable.)

To get what you want, you should use a smart pointer like shared_ptr (from either Boost, TR1, or C++0x), and completely disable copy-ability:

class ClassWithoutAss
{
public:
    const int mem;

    ClassWithoutAss(int mem):mem(mem){}
    // don't explicitly declare empty destructors

private:
    ClassWithoutAss(const ClassWithoutAss&); // not defined
    ClassWithoutAss& operator=(const ClassWithoutAss&); // not defined
};

typedef shared_ptr<ClassWithoutAss> ptr_type;

std::vector<ptr_type> vec;
vec.push_back(ptr_type(new ClassWithoutAss(1)));

Pointers can be copied just fine, and the smart pointer ensures you don't leak. In C++0x you can do this best with a std::unique_ptr, taking advantage of move-semantics. (You don't actually need shared semantics, but in C++03 it's easiest as it stands.)

Answer 2

The problem here is that types in a container must be assignable.

Because you do not define an assignment operator for your class the compiler will generate one for you. The default assignment operator will look like this:

ClassWithoutAss& operator=(ClassWithoutAss const& rhs)
{
    mem = copy.mem;
    return *this;
}
// The compiler generated assignemtn operator will copy all members
// using that members assignment operator.

In most situations this would work. But the member mem is a const and thus unassignable. Therefore compilation will fail when it tries to generate the assignment operator.