这是一个有问题的版本： 

class download_file :public QObject{Q_OBJECT;
public:download_file(QString url, QString path);void sendRequest();
public slots:void requestFinished(QNetworkReply* reply);
protected:QNetworkAccessManager* httpMgr;
private:QString url, path;
};

download_file::download_file(QString url,QString path)
{this->url = url;this->path = path;
}
void download_file::sendRequest()
{httpMgr = new QNetworkAccessManager(this);QObject::connect(httpMgr, SIGNAL(finished(QNetworkReply*)), this, SLOT(requestFinished(QNetworkReply*)));QNetworkRequest requestInfo;requestInfo.setUrl(QUrl(url));QNetworkReply *reply = httpMgr->get(requestInfo);
}
void download_file::requestFinished(QNetworkReply* reply)
{QVariant statusCode = reply->attribute(QNetworkRequest::HttpStatusCodeAttribute);if (statusCode.isValid())qDebug() << "status code=" << statusCode.toInt();QVariant reason = reply->attribute(QNetworkRequest::HttpReasonPhraseAttribute).toString();if (reason.isValid())qDebug() << "reason=" << reason.toString();QNetworkReply::NetworkError err = reply->error();if (err != QNetworkReply::NoError) {qDebug() << "Failed: " << reply->errorString();}else {// 获取返回内容QByteArray ba(reply->readAll());//qDebug() << ba;//保存QFile file(path);bool ok = file.open(QIODevice::WriteOnly);if(ok){file.write(ba);file.close();}else {qDebug() << "open failed!";}}
}

当要下载的文件过大时，程序会崩溃！！！(std::bad_alloc)（所以只可以下载一些小的文件）

我认为这是ba(QByteArray)太大了，要一下子往文件中写入太多的数据，导致new失败了。

修复：

Qt 基于http的网络文件下载_qt http下载文件-CSDN博客(参考+引用）

class download_file :public QObject{Q_OBJECT;
public:download_file(QString url, QString path);void sendRequest();
public slots:void on_finished();void on_readyRead();void on_downloadProgress(qint64 bytesRead, qint64 totalBytes);
protected:QNetworkAccessManager* httpMgr;
private:QString url, path;QFile* file;QNetworkReply* reply;
};

download_file::download_file(QString url,QString path)
{this->url = url;this->path = path;
}
void download_file::sendRequest()
{httpMgr = new QNetworkAccessManager(this);QNetworkRequest requestInfo;requestInfo.setUrl(QUrl(url));PRINTF_LOCATION() << url;reply = httpMgr->get(requestInfo);connect(reply, &QNetworkReply::finished, this, &download_file::on_finished);connect(reply, &QNetworkReply::readyRead, this, &download_file::on_readyRead);connect(reply, &QNetworkReply::downloadProgress, this, &download_file::on_downloadProgress);file = new QFile(path);if (!file->open(QIODevice::WriteOnly))return;}
void download_file::on_finished()
{file->close();delete file;file = nullptr;reply->deleteLater();reply = nullptr;
}
void download_file::on_downloadProgress(qint64 bytesRead, qint64 totalBytes)
{qDebug() << bytesRead << "   " << totalBytes;
}
void download_file::on_readyRead()
{file->write(reply->readAll());
}