Question

I had some problem with string arrays C++ array size different result I got the advice to use vectors instead of arrays. But this works:

#include "stdafx.h"
#include <string>
#include <iostream>
#include <vector>

using namespace std;

vector<int> a (1,2);

void test(vector<int> a)
{
    cout << a.size(); 
}
int _tmain(int argc, _TCHAR* argv[])
{
     test(a);

    return 0;
}

But this wont:

vector<string> a ("one", "two");

void test(vector<string> a)
{
    cout << a.size(); 
}
int _tmain(int argc, _TCHAR* argv[])
{
     test(a);

    return 0;
}

error C2664: ‘std::basic_string<_Elem,_Traits,_Ax>::basic_string(const std::basic_string<_Elem,_Traits,_Ax> &)’ : cannot convert parameter 1 from ‘const char’ to ‘const std::basic_string<_Elem,_Traits,_Ax> &’

I dont get whats going wrong.

Answer 1

The first one is calling a constructor (N, X) that creates N elements, each with a value of X, so you end up with one 2.

There is no match for the second constructor, as none take two const char * or similar.

Use curlies instead, as there is a match for an initializer list (at least in C++11):

std::vector<int> v{1, 2}; //contains a 1 and a 2
std::vector<std::string> v2{"hi", "bye"}; //contains "hi" and "bye"

In C++03, you can instead do this:

int vecInit[] = {1, 2, 3, 4};
std::vector<int> vec(vecInit, vecInit + sizeof vecInit / sizeof vecInit[0]);

You’ll end up copying the items from the array into the vector to initialize it because you utilize the constructor that takes two iterators, of which pointers are random-access.