how can i initialize a bi-dimensional vector in a class in C++?

Since i need to define its size in runtime, i need a vector. I tried the following but i get a compiling error: "error: ‘V’ is not a type"

#include<iostream>
#include<vector>

class graph {
private:
    int V;
    std::vector<int> row(V);
    std::vector<std::vector<int>> matrix(V,row);
public:
    graph(int v): V(v) {}
};

Is there something i am failing to understand? is it even allowed to initialize a vector this way?

>Solution :

The compiler considers these lines

std::vector<int> row(V);
std::vector<std::vector<int>> matrix(V,row);

as member function declarations with parameters with omitted type specifiers.

It seems what you need is the following

class graph {
private:
    int V;
    std::vector<std::vector<int>> matrix;
public:
    graph(int v): V(v),  matrix( v, std::vector<int>( v ) ) {}
};