On Dec 23, 4:25 am, Salt_Peter <pj_h...@yahoo.com> wrote:

On Dec 22, 11:37 am,

hi,

[re-arranged inline...] Do NOT Top Post

1 #include <vector>
2 #include <iostream>
3
4 struct A {
5 A(const unsigned int a) : u(a) {}
6 unsigned int u;
7 };
8
9 struct B {
10 B(const unsigned int a) : u(a) {}
11 B(const B& b) :u(b.u) {std::cerr<<" copy "<<std::endl;}
12 const unsigned int u;
13 };
14
15 int main () {
16
17 std::vector<A> p;
18 p.push_back(A(2)); // ok
19
20 B w(8); // ok
21 B x=w; // copy ok
22 B y(x); // copy ok
23 std::vector<B> z;
24 z.push_back(B(2)); // error
25 }

t.C: In member function 'B& B::operator=(const B&)':
t.C:9: instantiated from 'void std::vector<_Tp,
_Alloc>::_M_insert_aux(__gnu_cxx::__normal_iterator<typename
_Alloc::pointer, std::vector<_Tp, _Alloc> >, const _Tp&) [with _Tp =
B, _Alloc = std::allocator<B>]'
/usr/include/c++/4.0.0/bits/stl_vector.h:610: instantiated from
'void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = B,
_Alloc = std::allocator<B>]'
t.C:24: instantiated from here
t.C:9: error: non-static const member 'const unsigned int B::u', can't
use default assignment operator
/usr/include/c++/4.0.0/bits/vector.tcc: In member function 'void
std::vector<_Tp,
_Alloc>::_M_insert_aux(__gnu_cxx::__normal_iterator<typename
_Alloc::pointer, std::vector<_Tp, _Alloc> >, const _Tp&) [with _Tp =
B, _Alloc = std::allocator<B>]':
/usr/include/c++/4.0.0/bits/vector.tcc:260: warning: synthesized
method 'B& B::operator=(const B&)' first required here

On Dec 22, 9:47 am, Kira Yamato <kira...@earthlink.net> wrote:

On 2007-12-21 20:05:00 -0500, Salt_Peter <pj_h...@yahoo.com> said:

On Dec 21, 6:18 pm, Kira Yamato <kira...@earthlink.net> wrote:

On 2007-12-21 11:46:31 -0500, Salt_Peter <pj_h...@yahoo.com> said:

On Dec 21, 4:47 am, Kira Yamato <kira...@earthlink.net> wrote:

On 2007-12-21 03:38:33 -0500,

hi,

it seems i can't assign the const variable u in class A, one way to
solve the problem may be to build a copy constructor. however, why
does C++ or vector class not like this code? my g++ is: gcc version
4.0.1 (Apple Inc. build 5465). thanks for the help.

summary of compile error:
---------------------------------------
cpp.C:4: error: non-static const member 'const unsigned int A::u',
can't use default assignment operator
/usr/include/c++/4.0.0/bits/vector.tcc:260: warning: synthesized
method 'A& A::operator=(const A&)' first required here

code:
-------
1
2 #include <vector>
3
4 struct A {
5 A(const unsigned int a) : u(a) { }
6 private: const unsigned int u;
7 };
8
9 int main () {
10
11 std::vector<A> y;
12 y.push_back(A(2));
13 }

You have a number of problems with this code.

First, when (12) is executed, std::vector tries to allocate an array of
struct A. Unfortunately, struct A has no default constructor. So, it
cannot construct each element in the array.

There is no array, he's trying to copy into the empty vector a single
element.

Preciously. So, before line (12) can assign the element A(2) into
std::vector, the program must invoke code to allocate an array first.

How else will std::vector<A> store A(12)?

There is no array in an empty vector. The vector is empty.
pushing back a parametized element requires no default construction
either.
As i already mentioned, to have a vector invoke a default ctor you
would have to tell it to do so.

std::vector< A > v; // does not invoke any of A's ctors

std::vector< A > v(10); // invokes A's default ctor + 10 copies

A vector is not an array.
Here is the proof:

#include <iostream>
#include <vector>

class A
{
int m_n;
public:
A() : m_n(0) { std::cout << "A()\n"; }
A(int n) : m_n(n) { std::cout << "A(int)\n"; }
A(const A& copy)
{
std::cout << "A(const A& copy)\n";
m_n = copy.m_n;
}
};

int main()
{
std::cout << "declaring an empty std::vector:\n";
std::vector< A > v;
std::cout << "v.size() = " << v.size() << std::endl;

std::cout << "pushing an element:\n";
v.push_back( A(99) );
std::cout << "v.size() = " << v.size() << std::endl;

std::cout << "default constructing 10 elements:\n";
std::vector< A > v2(10);
std::cout << "v2.size() = " << v2.size() << std::endl;
}

/*
declaring an empty std::vector:
v.size() = 0
pushing an element:
A(int)
A(const A& copy)
v.size() = 1
default constructing 10 elements:
A()
A(const A& copy)
A(const A& copy)
A(const A& copy)
A(const A& copy)
A(const A& copy)
A(const A& copy)
A(const A& copy)
A(const A& copy)
A(const A& copy)
A(const A& copy)
v2.size() = 10
*/

i tried a copy constructor, but the problem persist.

a) Do not Top Post
b) try to reply to the appropriate message inline, help us help you
c) read the replies (it says you need op= and the compiler clearly
states that in error log too)
Ignore the compiler at your own peril

hi,

thanks for the help.

i noticed the compiler's log, if i define op= then i can't make the
variable "u" constant, if i don't make "u" const, then i don't have a
problem in the first place. how do i go around this issue?