将QUrl传递给QNetworkRequest构造函数会导致"非类型"编译器错误

Question

当我将QUrl传递给QNetworkRequest构造函数时,我从编译器中得到奇怪的错误.更奇怪的是,它仅在特定情况下发生,这是一个例子:

#include <QCoreApplication>
#include <QNetworkReply>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QString str;

    QNetworkRequest req(QUrl(str));
    req.setUrl(QUrl(str)); // error: request for member 'setUrl' in 'req', which is of non-class type 'QNetworkRequest ()(QUrl)'

    QNetworkRequest req2(QUrl(QString("http://google.com"))); // works here perfectly!
    req2.setUrl(QUrl(str));

    QNetworkRequest req3;
    req3.setUrl(QUrl(str));

    QUrl url(str);
    QNetworkRequest req4(url);
    req4.setUrl(QUrl(str));

    return a.exec();
}

g ++ - 4.2说:

main.cpp:11: error: request for member 'setUrl' in 'req', which is of non-class type 'QNetworkRequest ()(QUrl)'

vs2010编译器说:

error C2228: left of '.setUrl' must have class/struct/union

当我想将"损坏的"QNetworkRequest传递给QNetworkAccessManager :: get方法时,我会从vs2010获得一个额外的解释:

error C2664: 'QNetworkAccessManager::get' : cannot convert parameter 1 from 'QNetworkRequest (__cdecl *)(QUrl)' to 'const QNetworkRequest &'
      Reason: cannot convert from 'overloaded-function' to 'const QNetworkRequest'
      No constructor could take the source type, or constructor overload resolution was ambiguous

多奇怪..谁有线索？也许我只是忘了如何编程..

Answer 1

你遇到了一个所谓的"烦恼的解析".这个:

QNetworkRequest req(QUrl(str));

相当于

QNetworkRequest req(QUrl str);

这是一个函数声明.

要声明变量并从临时初始化它,您需要

QNetworkRequest req{QUrl{str}};                    // C++11
QNetworkRequest req = QNetworkRequest(QUrl(str));  // Historic C++ (if it's copyable)

或者使用命名变量而不是临时变量req4.