IntelliSense: no operator "<<" matches these operands

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C++

Visual-Studio

Hi,

IntelliSense: no operator "<<" matches these operands
operand types are: std::ostream << std::string c:\Users\mohammad\Documents\Visual Studio 2013\Projects\summing a list of number\summing a list of number\summing a list of number.cpp 10

here is the Code:

// summing a list of number.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include "iostream"
using namespace std;
int sum(int a[], int from, int size, const string& context, int depth)
{
	string indent(depth, '|');
	cout << indent << context << "(a, " << from << ", " << size << ")" << endl;
	int result = 0;
	if (size == 1)
	{
		result = a[from];
	}
	else if (size > 1)
	{
		int midpoint = size / 2;
		int left = sum(a, from, midpoint, "left", depth + 1);
		int right = sum(a, from + midpoint, size - midpoint, "right", depth + 1);
		result = left + right;
		cout << indent << "=" << left << "+" << right << endl;
	}
	cout << indent << "=" << result << endl;
	return result;
}

int main(){
	int a[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
	cout << "sum = " << sum(a, 0, 10, "sum", 0) << endl;
	getchar();
}

why does it make an error out of std and ostream, while I already have included iostream and std ?

(I'm using VS-2013)