C ++二进制操作数无效

Question

我是 C++ 的初学者，我遇到了这段代码的问题，它应该在 Superbowl 决赛期间显示分数：

#include <iostream>

enum POINTS { EXTRA_POINT = 1, SAFETY = 2, FIELD_GOAL = 3, TOUCHDOWN =6 };

unsigned short giantsScore = 0, patriotsScore = 0;

int main()
{
std::cout << " Giants: " << giantsScore << "\n";
std::cout << " Patriots: " << patriotsScore << "\n\n";

std::cout << " Giants: " << giantsScore = giantsScore + SAFETY << "\n";
std::cout << " Patriots: " << patriotsScore << "\n\n"; 

std::cout << " Giants: " << giantsScore = giantsScore + TOUCHDOWN + EXTRA_POINT << "\n";
std::cout << " Patriots: " << patriotsScore << "\n\n";

std::cout << " Giants: " << giantsScore << "\n";
std::cout << " Patriots: " << patriotsScore = patriotsScore + FIELD_GOAL << "\n\n";

std::cout << " Giants: " << giantsScore << "\n";
std::cout << " Patriots: " << patriotsScore = patriotsScore + TOUCHDOWN + EXTRA_POINT << "\n\n";

std::cout << " Giants: " << giantsScore << "\n";
std::cout << " Patriots: " << patriotsScore = patriotsScore + TOUCHDOWN + EXTRA_POINT  << "\n\n";

std::cout << " Giants: " << giantsScore = giantsScore + FIELD_GOAL << "\n";
std::cout << " Patriots: " << patriotsScore << "\n\n";

std::cout << " Giants: " <<  giantsScore = giantsScore + FIELD_GOAL << "\n";
std::cout << " Patriots: " << patriotsScore << "\n\n";

std::cout << " Giants: " << giantsScore << "\n";
std::cout << " Patriots: " << patriotsScore = patriotsScore + FIELD_GOAL + EXTRA_POINT  << "\n\n";

return 0;
}

忽略这很不优雅，当我通过编译器 G++ 运行它时，我收到错误消息

错误：'int'和'const char [2]'类型的无效操作数到二进制'operator

如果我删除常量并将它们添加到每个std::cout 之前，那么它运行良好。我只是想知道为什么我不能在每个输出行中添加常量？

Answer 1

您的错误消息指出：int << char，这当然是一个奇怪的操作。

这是因为运营商的优先级。

每个运算符都有一个优先级，这意味着它将在评估其他运算符之前或之后进行评估。

+ 在= 之前计算

并且<< 应该在= 具有cout<<"stuff" 的原始目的之后进行评估。

<< 最初是位移运算符（现在仍然是），所以这就是您遇到这种奇怪行为的原因。添加括号，你会很好。

Answer 2

查看http://cs.smu.ca/~porter/csc/ref/cpp_operators.html 了解运算符优先级规则的概述。当你写这个时：

std::cout << " Patriots: " << patriotsScore = patriotsScore + FIELD_GOAL + EXTRA_POINT  << "\n\n";

那么根据优先级规则，+操作符会先被执行，给你这个：

std::cout << " Patriots: " << patriotsScore = result  << "\n\n";

然后执行

要解决您的问题，请在赋值操作周围加上括号，如下所示：

std::cout << " Patriots: " << (patriotsScore = patriotsScore + FIELD_GOAL + EXTRA_POINT)  << "\n\n";