自定义操纵器使用 Visual C++ 而非 g++/clang 进行编译

Question

我有一个记录器 class QueuedLog，它对日志消息进行排队，并在需要时将队列中的所有日志消息插入 std::ostream。为了分隔每条日志消息，我编写了一个名为 endm 的操纵器，其使用方式与 std::endl 类似。例如，这是一个用例：

QueuedLog log(INFO);

// do stuff

log << "test: 0x" << std::hex << std::uppercase << 15 << endm; // add a log message

// do more stuff

log << "log something else" << endm;

std::cout << log << std::endl; // insert all the queued up log messages into cout

// do more stuff

log << "start another message...";

// calculate something, add it to a log message and end the message
log << std::dec << 42 << endm;

std::cout << log << std::endl; // insert all the queued up log messages into cout

log << "add yet another message" << endm;

// don't need to log that last message after all
// leave it in the QueuedLog instead of inserting it into cout

我的代码可以用 Visual C++ 很好地编译，但是当我尝试使用 endm 操纵器时，g++ 和 clang++ 编译失败。这是 QueuedLog 的最小版本（通常驻留在单独的头文件中），使用一个小示例来说明问题：

#include <ios>
#include <iostream>
#include <string>
#include <sstream>
#include <deque>
#include <stdexcept>

namespace Logger {

enum LogType {
    NONE,   
    DEBUG,  
    INFO,   
    WARN,   
    ERROR,  
    FATAL   
};

// Converts a LogType to a `std::string` which can be prepended to a log message.
std::string prepend_type(LogType type) {
    switch (type) {
        case DEBUG: return std::string("[DEBUG] ");
        case INFO: return std::string("[INFO] ");
        case WARN: return std::string("[WARN] ");
        case ERROR: return std::string("[ERROR] ");
        case FATAL: return std::string("[FATAL] ");
        default: return std::string("");
    }
}

class QueuedLog {
    /* Holds a partially contructed log message.

    A std::stringstream is used instead of a std::string so that non-string data types can be inserted into
    the QueuedLog without requiring conversion. Also, client code can apply I/O manipulators like std::hex.
    */
    std::ostringstream stream;

    std::deque<std::string> messages; // Holds the queued, completed log message(s).

    // The LogType of new messages inserted into the QueuedLog. This can be changed at any time.
    LogType log_type;
public:
    QueuedLog(LogType logtype = NONE) : log_type(logtype) {} // Constructs a QueuedLog with no text and an initial LogType.

    // Inserts a character sequence into the QueuedLog.
    template<typename T> inline QueuedLog& operator<<(const T& message) {
    //inline QueuedLog& operator<<(const std::string& message) { // non-template version doesn't work, either
        // Only prepend with logtype if it is the beginning of the message
        if (stream.str().empty()) stream << prepend_type(log_type);

        stream << message;
        return *this;
    }

    // Overload used for manipulators like QueuedLog::endm()
    inline QueuedLog& operator<<(QueuedLog& (*pf)(QueuedLog&)) {
        (*pf)(*this);
        return *this;
    }

    // Adds the newline character and marks the end of a log message.
    friend inline QueuedLog& endm(QueuedLog& log) {
        log.stream << log.stream.widen('\n');

        // Add the completed message to the messages deque, and reset the stream for the next message
        log.messages.push_back(log.stream.str());
        log.stream.str(""); // clear the underlying string
        log.stream.clear(); // clear any error flags on the stream

        return log;
    }

    /* Inserts all the completed log messages in the QueuedLog object into a std::ostream.

    If the QueuedLog contains an incomplete log message (a message that has not been terminated by QueuedLog::endm())
    then that partial message will not be inserted into the std::ostream.
    */
    friend inline std::ostream& operator<<(std::ostream& os, QueuedLog& log) {
        while (!log.messages.empty()) {
            os << log.messages.front();
            log.messages.pop_front();
        }

        return os;
    }
};

} // end namespace Logger

using namespace Logger;

int main() {
    QueuedLog log(INFO);

    log << "test: 0x" << std::hex << std::uppercase << 15; // compiles by itself with all compilers
    log << endm; // but compilation error w/ g++/clang++ when trying to use endm
    std::cout << log << std::endl;
}

或者，这个（可能过度）简化的例子：

class QueuedLog {
public:
    friend inline void endm() {
    }
};

int main() {
    endm;
}

我试图用 rextester 的所有三个编译器编译它，但它只能用 Visual C++ 成功编译。

g++ 给出以下错误：

error: ‘endm’ was not declared in this scope

clang++的报错信息类似：

error: use of undeclared identifier 'endm'

为什么这在 Visual C++ 中有效，但在 g++ 或 clang++ 中无效？我如何为 g++/clang++ 修复它？该解决方案不需要同时在所有三个编译器中工作，我只想知道如何为 g++ 和 clang++ 修复它。

Answer 1

在 class 定义中定义的友元函数仅在 class 定义中可见。您需要在 class 定义之外声明函数。在 class QueuedLog 之外添加此行，但在 namespace Logger:

之内

extern QueuedLog& endm(QueuedLog&);

自定义操纵器使用 Visual C++ 而非 g++/clang 进行编译

Custom manipulator compiles with Visual C++ but not g++/clang

c++

visual-c++

g++

clang++

manipulators