"Variable was not declared in this scope" in C and C++

I find a difference between C and C++ today. I separated the program in two different files, this is my C test:

/* in file main.c*/
#include <stdio.h>

int main()
{
    int a = 3, b = 4;
    int c = sum(a, b);
    printf("%d\n", c);
}

and

/* In file sum.c */
#include <stdio.h>

int sum(int x, int y)
{
    return x + y;
}

then, I compiled them gcc main.c sum.c， no errors and the result is correct. Following is my C++ test, I also separated them in two different files:

/* in file main.cpp*/
#include <iostream>

int main()
{
    int a = 3, b = 4;
    int c = sum(a, b);
    std::cout << c << std::endl;
}

and

/* In file sum.cpp */
#include <iostream>

int sum(int x, int y)
{
    return x + y;
}

Compile them g++ main.cpp sum.cpp. An error occurs: error: ‘sum’ was not declared in this scope. If I put a declaration int sum(int, int) in the file main.cpp, then no error occurs. Why there is such a different between C and C++? What's the best way to solve it?