错误：开关数量不是整数 [英] error: switch quantity not an integer

问题描述

我已经研究了我的问题在StackOverflow和多谷歌的链接，我仍然困惑。我认为最好的东西对我来说是问...

I have researched my issue all over StackOverflow and multi-google links, and I am still confused. I figured the best thing for me is ask...

我创建一个简单的命令行计算器。这里是我的代码到目前为止：

Im creating a simple command line calculator. Here is my code so far:

const std::string Calculator::SIN("sin");  
const std::string Calculator::COS("cos");  
const std::string Calculator::TAN("tan");  
const std::string Calculator::LOG( "log" );  
const std::string Calculator::LOG10( "log10" );

void Calculator::set_command( std::string cmd ) {

    for(unsigned i = 0; i < cmd.length(); i++)
    {
    cmd[i] = tolower(cmd[i]);
    }

    command = cmd;
}

bool Calculator::is_legal_command() const {

    switch(command)
    {
    case TAN:
    case SIN:
    case COS:
    case LOG:
    case LOG10:
        return true;
        break;
    default:
        return false;
        break;
    }

}

我得到的错误是： / p>

the error i get is:

Calculator.cpp: In member function 'bool Calculator::is_trig_command() const':  
Calculator.cpp: error: switch quantity not an integer  
Calculator.cpp: error: 'Calculator::TAN' cannot appear in a constant-expression  
Calculator.cpp: error: 'Calculator::SIN' cannot appear in a constant-expression  
Calculator.cpp: error: 'Calculator::COS' cannot appear in a constant-expression  

强大的互联网，它说明字符串可以在switch语句中使用。

The mighty internet, it says strings are allowed to be used in switch statements.

感谢大家，感谢您的帮助。

Thanks everyone, I appreciate your help.

推荐答案

在切换中，表达式必须是a 整合类型或类型类型，其中有明确的转换为整数类型（引用VS2008 docs ）。

In switch, the expression must be of "an integral type or of a class type for which there is an unambiguous conversion to integral type" (quoting VS2008 docs).

字符串类没有明确转换为整数类型，如 char 。

A string class doesn't have "unambiguous conversion to integral type", like a char does.

解决方法：

1. 创建地图< string，int> 并打开地图的值： switch（command_map [command]）
   `

1. Create a map<string, int> and switch on the value of the map: switch(command_map[command]) `

执行如果 / else 而不是switch。

Do a set of if/else instead of switch. Much more annoying and hard to read, so I'd recommend the map route.

另外，一个偶数对于真正复杂的逻辑，更好的解决方案是改进映射解决方案，以摆脱开关完全，而是与一个函数查找： std :: map< std :: string，functionPointerType> 。它可能不需要您的特定情况，但是对于复杂的很长的查找逻辑更快。

As an aside, an even better solution for really complicated logic like that is to improve the mapping solution to get rid of switch completely and instead go with a function lookup: std::map<std::string, functionPointerType>. It may not be needed for your specific case, but is MUCH faster for complicated very long look-up logic.

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