How to initialize vector size inside a constructor?

Question

How to initialize a vector in C++ to some specific size inside constructor with c++11? Something like this (vector called my_vec):

#ifndef MAP_H
#define MAP_H

#include <vector>

struct color
{
    int r;
    int g;
    int b;
};

class map
{
    private:
        int gridSize;
        int verticalNum;
        int horizontalNum;
        std::vector< std::vector<color> > my_vec(100, vector<color>(100));
    public:
        map();
        ~map();
};

#endif // MAP_H

This doesn`t work so I tried this:

#ifndef MAP_H
#define MAP_H

#include <vector>

struct color
{
    int r;
    int g;
    int b;
};

class map
{
    private:
        int gridSize;
        int verticalNum;
        int horizontalNum;
        std::vector< std::vector<color> > *my_vec;
    public:
        map();
        ~map();
};

#endif // MAP_H

and in constructor I tried to initialize it like:

map::map()
{
    this->my_vec(100, std::vector<color>(100));
}

But I get an error that : "expression cannot be used as a function"

Answer 1

You have to do it in the constructor's initializer list:

map::map() : my_vec(100, std::vector<color>(100))
{

}

But you could also initialize it as an in class variable as:

class MyMap {
  // ...
  std::vector< std::vector<color> > my_vec = 
    std::vector<std::vector<color>>(100, std::vector<color>(100));
public:
  MyMap() {}
  ~MyMap() {}
};

Live Demo

Answer 2

As you just specified C++11, in-class initialzation may be exactly what you want:

private:
    std::vector<std::vector<color>> my_vec = std::vector<std::vector<color>>(100, std::vector<color>(100));

Answer 3

You need to use a constructor initializer list

map::map()
    : my_vec(100, std::vector<color>(100))
{}

However, if the actual code have a compile-time hard-coded size, why not use std::array instead? Like

std::array<std::array<color, 100>, 100> my_vec;