Nested function is supported as an extension in gcc, but not g++.

NestedFunc.c

#include <stdio.h>

double foo(double a, double b)
{
	double square(double a) { return a * a; }

	return square(a) + square(b);
}

int main(int argc, char **argv)
{
	int a = 2, b = 3;
	printf("%lf\n", foo(a, b));
	return 0;
}