Re: Array inside struct inside container

Victor Bazarov <v.bazarov@comcast.invalid>
Tue, 16 Aug 2011 15:28:41 -0400
On 8/16/2011 3:12 PM, none wrote:

I've seen forum posts here and there asking "How do I create an STL
container of arrays?" Or, equivalently, "How do I fix this syntax:"

    std::vector<float[20]> v;

The answers to these questions are generally one of two things:

1) Use a boost::array, or
2) Put the array in a struct, like so:

    // In header file
    stuct wrapper { float f[20]; };

    // In CPP file
    std::vector<wrapper> v;

I was under the impression that things stored in STL containers needed to
be copyable and assignable. That second option seems incorrect to me,
unless the compiler is expected to generate both an operator= and a copy
constructor for the struct that perform a memcpy() for the contents of f.

Not memcpy. See below.

Is it?

A struct with an array as a data member is assignable. The default
compiler-provided assignment operator performs memberwise assignment
that for array members means element-wise assignment. You can verify it
by compiling/running this test program:

    #include <iostream>

    struct A {
      A& operator=(A const&) { std::cout << "A::op=\n"; return *this; }
    struct B { A aa[10]; };

    int main() {
        B b1, b2;
        b1 = b2;

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