c++ - 有没有更短的写法 `StringPtr ? StringPtr : "null"`？

Question

我有这个代码:

 std::wstringstream outstream;
 outstream << (prop.m_pwszOriginalVolumeName 
             ? prop.m_pwszOriginalVolumeName 
             : L"null") << L";"
           << (prop.m_pwszSnapshotDeviceObject 
             ? prop.m_pwszSnapshotDeviceObject 
             : L"null") << L";"
           << (prop.m_pwszOriginatingMachine 
             ? prop.m_pwszOriginatingMachine
             : L"null") << L";"
           << ... // some more strings here

有没有办法既避免代码重复，又保持代码简洁？

Answer 1

你可以定义一个小函数:

whatever_t strOrNull(whatever_t str) {
    return str ? str : L"null";
}

那么你的代码就变成了

std::wstringstream outstream;
outstream << strOrNull(prop.m_pwszOriginalVolumeName)   << L";"
          << strOrNull(prop.m_pwszSnapshotDeviceObject) << L";"
          << strOrNull(prop.m_pwszOriginatingMachine)   << L";"
          << ... // some more strings here

或者如果你想更简洁，你可以这样做(取决于 whatever_t 是什么；如果 wstringstream 已经有该类型的 operator<< 重载，这将不起作用):

wstringstream& operator<<(wstringstream& out, whatever_t str) {
    if (str)
        out << str;
    else
        out << L"null";

    return out;
}

那么你的代码就变成了

std::wstringstream outstream;
outstream << prop.m_pwszOriginalVolumeName   << L";"
          << prop.m_pwszSnapshotDeviceObject << L";"
          << prop.m_pwszOriginatingMachine   << L";"
          << ... // some more strings here