程序查错——参数引用

#include <iostream>using namespace std;const float pi = 3.14f;float f;float f1(float r){    f = r*r*pi;    return f;}float& f2(float r){    f = r*r*pi;    return f;}int main(void){    float f1(float = 5);    float& f2(float = 5);    float a = f1();    float& b = f1();//错    float c = f2();    float& d = f2();}