C++ 将成员函数指针保存到 class 变量

Question

我是新手 C++ 程序员，所以这可能会变得混乱。

我正在编写一个程序来将二进制 Windows ETL 文件转换为 Wireshark PCAPNG。更准确地说，我正在修复由于 ETL 格式更改而出现的错误。为了修复这个错误，我需要在我的代码中添加一个回调。以下是代码中的相关部分：

#include <iostream>
#include <fstream>
#include <functional>

// Ng Block Buffer
typedef struct _NG_BLK_BUFFER {
    int blockType = 0;

    // ...
} NG_BLK_BUFFER;

class RingBuffer
{
public:
    // ...
    std::function<void(NG_BLK_BUFFER*)> framePayloadWriter;
};

class PcapNgFile : public std::ofstream
{
public:
    // ...
    PcapNgFile(const std::wstring fileName, size_t ringBufferSize);

    void writeEPB();
    // ...

    RingBuffer* ringBuffer;
};

PcapNgFile::PcapNgFile(const std::wstring fileName, size_t ringBufferSize)
{
    // ...
    this->ringBuffer = new RingBuffer();

    this->ringBuffer->framePayloadWriter = std::bind(&PcapNgFile::writeEPB, this, std::placeholders::_1);
}

void PcapNgFile::writeEPB()
{
    std::cout << "Writing an EPB" << std::endl;
}

int wmain(int argc, wchar_t** argv)
{
    PcapNgFile* myPcapNg = new PcapNgFile(L"test.pcap", 1);
}

对于第 36 行，我得到错误：

Error   C2679   binary '=': no operator found which takes a right-hand operand of type 'std::_Binder<std::_Unforced,void (__thiscall PcapNgFile::* )(void),PcapNgFile *,const std::_Ph<1> &>' (or there is no acceptable conversion)

第 36 行是：

this->ringBuffer->framePayloadWriter = std::bind(&PcapNgFile::writeEPB, this, std::placeholders::_1);

我做错了什么？

Answer 1

writeEPB 不带参数，但您试图将其绑定到带参数的函数，将其更改为 writeEPB(NG_BLK_BUFFER*).

如果您想忽略回调中的参数，请从绑定调用中删除占位符：

this->ringBuffer->framePayloadWriter = std::bind(&PcapNgFile::writeEPB, this);

使用 lambda 而不是 std::bind 可能稍微更有效并且往往会提供更好的错误消息。例如：

this->ringBuffer->framePayloadWriter = [this](){ writeEPB(); };

给出更容易理解的错误信息，正确的 lambda 应该是：

this->ringBuffer->framePayloadWriter = [this](NG_BLK_BUFFER*){ writeEPB(); };

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