C++ 通过引用返回值

使用引用而不是指针可以使C++程序更易读和维护。类似于返回指针，C++函数也可以返回引用。

当函数返回一个引用时，实际上返回的是其返回值的隐式指针。这样，函数可以在赋值语句的左侧使用。例如，考虑下面这个简单的程序−

#include <iostream>
#include <ctime>

using namespace std;

double vals[] = {10.1, 12.6, 33.1, 24.1, 50.0};

double& setValues( int i ) {
   return vals[i];   // return a reference to the ith element
}

// main function to call above defined function.
int main () {

   cout << "Value before change" << endl;
   for ( int i = 0; i < 5; i++ ) {
      cout << "vals[" << i << "] = ";
      cout << vals[i] << endl;
   }

   setValues(1) = 20.23; // change 2nd element
   setValues(3) = 70.8;  // change 4th element

   cout << "Value after change" << endl;
   for ( int i = 0; i < 5; i++ ) {
      cout << "vals[" << i << "] = ";
      cout << vals[i] << endl;
   }
   return 0;
}

当上述代码编译并执行时，会产生以下结果−

Value before change
vals[0] = 10.1
vals[1] = 12.6
vals[2] = 33.1
vals[3] = 24.1
vals[4] = 50
Value after change
vals[0] = 10.1
vals[1] = 20.23
vals[2] = 33.1
vals[3] = 70.8
vals[4] = 50

当返回一个引用时，要注意所引用的对象不要超出作用域。因此，返回对局部变量的引用是不合法的。但你可以始终返回对静态变量的引用。

int& func() {
   int q;
   //! return q; // Compile time error
   static int x;
   return x;     // Safe, x lives outside this scope
}