Vector of structs with const members?

Question

Let's say I have

#include <string>
#include <vector>
using namespace std;

struct Student
{
    const string name;
    int grade;
    Student(const string &name) : name(name) { }
};

How do I, then, keep a vector of students?

int main()
{
    vector<Student> v;

    // error C2582: 'operator =' function is unavailable in 'Student'
    v.push_back(Student("john"));
}

Is there even a way to do this, or must I allocate all the students on the heap, and store a pointer to each of them instead?

Answer 1

You can't. Your type violates the "Assignable" requirement for standard containers.

ISO/IEC 14882:2003 23.1 [lib.container.requirements] / 3:

The type of objects stored in these components must meet the requirements of CopyConstructible types (20.1.3), and the additional requirements of Assignable types.

From table 64 (Assignable requirements):

In Table 64, T is the type used to instantiate the container, t is a value of T, and u is a value of (possibly const) T.

expression: t = u; return type: T; post-condition: t is equivalent to u

In theory, a std::vector equivalent could choose to do destruction and copy construction in all cases, but that's not the contract that has been chosen. If reallocation isn't required, then using the contained type's assignment operator for things like vector::operator= and vector::assign might be significantly more efficient.

Answer 2

The simple answer is: you can't. If you have const member variables, then the compiler can't supply a default copy-assignment operator. However, many of the operations that std::vector provides need to make assignments, and therefore require a (public) copy-assignment operator.

Your options are:

1. Make name non-const.
2. Write your own copy-assignment operator, and think of a way to deal with "copying" a const member.