自定义运算符支持 std::hex <<

Question

我在我的

operator<<

类中编写了自己的

Logger

实现：

#pragma once

#include <map>
#include <vector>
#include <string>

class Logger
{
public:

    enum Severity
    {
        INFORMATION = 0,
        WARNING = 1,
        ERROR = 2
    };

    Logger( std::ostream &os, Logger::Severity severity )
    : outputStream( os ), loggerSeverity( severity ) {}

    const std::vector<std::string>& getLogLines() const { return logLines; }

    friend Logger& operator<<( Logger &logger, Logger::Severity severity ) {
        logger.currentSeverity = severity;
        logger.outputStream << logger.severityMap.find( severity )->second << ": ";
        return logger;
    }

    friend Logger& operator<<( Logger &logger, const char *logLine ) {
        if( logger.currentSeverity >= logger.loggerSeverity )
        {
            logger.logLines.push_back( logLine );
            logger.outputStream << logLine;
        }
        return logger;
    }

private:
    std::ostream &outputStream;
    Logger::Severity loggerSeverity;
    std::vector<std::string> logLines;
    Logger::Severity currentSeverity;
    const std::map<Logger::Severity, std::string> severityMap =
    {
        { Logger::Severity::INFORMATION, "<INF>" },
        { Logger::Severity::WARNING, "<WARN>" },
        { Logger::Severity::ERROR, "<ERR>" }
    };
};

现在我想知道如何支持

std::hex

格式？我希望能够像这样调用我的记录器：

logger << Logger::Severity::ERROR << "CRC mismatch ( expected 0x" << std::hex << record->metaData.crc << " but got 0x" << std::hex << crc32 << std::dec << " )" << "\n";

但我得到：

error: no match for ‘operator<<’ (operand types are ‘Logger’ and ‘std::ios_base&(std::ios_base&)’)
   79 |         logger << Logger::Severity::ERROR << "CRC mismatch ( expected 0x" << std::hex << record->metaData.crc << " but got 0x" << std::hex << crc32 << std::dec << " )" << "\n";
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^~ ~~~~~~~~
      |                                           |                                       |
      |                                           Logger                                  std::ios_base&(std::ios_base&)

Answer 1

std::ostream

是如何做到的？

std::hex 是一个带有签名的函数“操纵器”

std::ios_base& hex( std::ios_base& str );

另请参阅 https://en.cppreference.com/w/cpp/io/manip/hex

并且 ostream 对于“输出”函数有一个重载，如下所示：

basic_ostream& operator<<( std::ios_base& (*func)(std::ios_base&) );

另请参阅 https://en.cppreference.com/w/cpp/io/basic_ostream/operator_ltlt

下的 (18)

您可以为此类型定义自己的运算符

<<

，将其传递到底层流。

如果你想自己处理它，你需要一些

std::ios_base

类型的东西，你可以给操纵器作为参数，它会做相当于

.setf(std::ios_base::hex, std::ios_base::basefield)

的事情。

或者你必须发明自己的操纵器而不是使用标准操纵器。