Question about storing array in a std::vector in C++

Question

I am unclear about the following.

First, this code compiles fine:

#include 

typedef struct{
    int x1,x2,x3,x4;
}  ints;

typedef std         


        

Answer 1

error: ISO C++ forbids initialization in array new
error: array must be initialized with a brace-enclosed initializer
error: invalid array assignment
error: request for member ‘~int [4]’ in ‘* __p’, which is of non-class type ‘int [4]’

To understand one of the errors, imagine the following:

void main() {
    int a[4] = {0,1,2,3};
    int b[4] = a;
}

As opposed to:

typedef struct{
    int x1,x2,x3,x4;
}  ints;

int main()
{
    ints a;
    ints b = a;
}

Or even:

typedef struct{
    int x[4];
}  ints;

int main()
{
    ints a;
    ints b = a;
}

C/C++ arrays cannot be copied via the assignment operator, though structs containing arrays can be.
So an easy fix is to do:

typedef struct{
        int x[4];
}  ints;

typedef std::vector<ints> vec;

int main(){
        vec v;
        ints a = { {0,1,2,3} };
        v.push_back(a);
}

Answer 2

The requirement for value type T for all STL containers, including std::vector<T>, is that T is Assignable - ISO C++03 23.1[lib.container.requirements]/4-5. Assignable is defined as follows:

Expression t = u, where t is of type T, and u is of type cv T, is valid, its return type is T&, and the post-condition is that t is equivalent to u.

Arrays do not fulfill this requirement, because you cannot write:

int a[2], b[2];
a = b;

The reason why you cannot is because both a and b in the code snippet above decay to pointer-type rvalues according to the usual C++ rules for array-to-pointer decay described in 4.2[conv.array]. Naturally, an rvalue if not permitted on the left side of non-overloaded operator=.

Answer 3

Under the hood it's doing an assignment and that isn't defined for arrays.

The pertinent part of the error is

instantiated from here /usr/lib/gcc/i586-redhat-linux/4.4.1/../../../../include/c++/4.4.1/bits/vector.tcc:306: error: array must be initialized with a brace-enclosed initializer /usr/lib/gcc/i586-redhat-linux/4.4.1/../../../../include/c++/4.4.1/bits/stl_vector.h:741: instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = int [4], _Alloc = std::allocator]’ test2.cpp:9: instantiated from here /usr/lib/gcc/i586-redhat-linux/4.4.1/../../../../include/c++/4.4.1/bits/vector.tcc:312: error: invalid array assignment

Answer 4

Try boost::array instead of plain arrays. It provides STL-compliant interface around fixed-size arrays, so it can be used inside STL containers. Plus, it implements boundary checking (boost::array::at).

#include <boost/array.hpp>
#include <vector>

typedef std::vector< boost::array<int, 4> > vec;
int main(){
    vec v;
    boost::array<int, 4> va = {0,1,2,3};
    v.push_back(va);
}

Answer 5

Arrays are not first class in C++; you cannot pass them as arguments to functions, for example (they decay to pointers, although you can pass pointers and references to arrays). Further, they do not have value semantics.

Answer 6

It's been a little while since I used C++, but I believe the core problem you're encountering is that arrays don't have the required semantics to get along well with a std::vector<>. I don't have my copy of Stroustrup handy, or I'd give you a reference.