问题
I am using Qt but this is a generic C++ question. My case is simple, I have a class Constants
which has a constant static member which I want it to be initialized after certain function calls are made.
Constants.h
#ifndef CONSTANTS_H
#define CONSTANTS_H
class Constants
{
public:
static const char* const FILE_NAME;
};
#endif // CONSTANTS_H
Constants.cpp
#include "constants.h"
#include <QApplication>
const char* const Constants::FILE_NAME = QApplication::applicationFilePath().toStdString().c_str();
main.cpp
#include <QtGui/QApplication>
#include "mainwindow.h"
#include "constants.h"
#include <QDebug>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
qDebug()<< "name: "<<Constants::FILE_NAME;
//for those who are unfamiliar with Qt, qDebug just prints out
return a.exec();
}
When compiling I got:
QCoreApplication::applicationFilePath: Please instantiate the QApplication object first
So problem here is obvious. When QApplication's static function is called in Constants.cpp QApplication is not installed by Qt yet. I need to somehow wait until QApplication a(argc, argv);
line is passed in main.cpp
is it possible and if not what else could you suggest to overcome this?
thanks
回答1:
One option is to return it from a function, keeping it in a static variable. This will be initialised when the function is first called.
char const * const file_name()
{
// Store the string, NOT the pointer to a temporary string's contents
static std::string const file_name =
QApplication::applicationFilePath().toStdString();
return file_name.c_str();
}
回答2:
Typical solution:
#ifndef CONSTANTS_H
#define CONSTANTS_H
class Constants
{
public:
static const char* const getFILE_NAME();
};
#endif // CONSTANTS_H
And in cpp
#include "constants.h"
#include <QApplication>
const char* const Constants::getFILE_NAME()
{
static const char* const s_FILE_NAME = QApplication::applicationFilePath().toStdString().c_str();
return s_FILE_NAME;
}
来源:https://stackoverflow.com/questions/7806589/c-is-it-possible-to-delay-initialization-of-constant-static-member