why using std::sqrt failed if I redefine the sqrt function?

Question

#include<iostream>

double sqrt(double);

int main()
{
    double a = std::sqrt(4.0);
    std::cout << a; 
    return 0;
}

double sqrt(double a)
{
    return 1.0;
}

I know I declare the sqrt at first, but I use std::sqrt, it still call my own sqrt. why?

Answer 1

Search your code for a using namespace std; somewhere. Perhaps hidden in some other set of include files.