when is a QString not really as QString - argument rejected by QFile

[bitbasher]

hi folks

this code (Qt V6, Windows 11)

bool Game::saveGame(const QString filename, const FileFormat format) const {
    QFile fn(filename);

is driving me nuts. As you can see i want to save data into a json file using the name in argument "filename"

Visual Studio Code is reporting that QFile does not have a constructor that matches this argument list and that the argument is of type QString .. so .. the error might even be in VSC ??

i have also tried filename.ToStdString() to see if it would use the QFile constructor that will take the file name that way .. same problem

I tried making a file info QFileInfor infoname(filename) .. that works .. but then
QFile nf( infoname.FilePath() ) still has no constructor for a QString

help?

[hskoglund]

And I'm guessing

QFile fn;
fn.setFileName(filename);

gives you the same error?

[bitbasher]

yes .. except the error flag is on the name of the QFile, "fn" .. but .. GitHUb CoPilot found a solution .. if i can screen shot the answer

i changed the code to

// Save Game object to file
bool Game::saveGame( const QString filename, const FileFormat format) const {
    QFile fn();
    fn.setFileName( filename );
    if( fn.exists() ) {
        std::cerr << std::string("Error: File already exists: ") << fn.fileName().toStdString() << std::endl;
        return false;
    }

and that changed the message to this

expression must have class type but it has type "QFile (*)()"

and now all the reference to "fn" are flagged as errors

it took a beat but the AI Proposed this as the root of the problem

The problem is that QFile fn() is being interpreted as a function declaration rather than object construction. Remove the parentheses to properly declare a QFile object.

and it changed the code to this

so you see any difference? cause i do not .. but it is fixed so ..

[Christian Ehrlicher]

@bitbasher said in when is a QString not really as QString - argument rejected by QFile:

so you see any difference? cause i do not .. but it is fixed so ..

QFile fn();

--> function call

QFile fn;

--> ctor

[Christian Ehrlicher]

@bitbasher said in when is a QString not really as QString - argument rejected by QFile:

the error might even be in VSC ??

Does the compiler complain or Visual studio code?

bool saveGame(const QString filename) {
    QFile fn(filename);
    return fn.exists();
}

This compiles fine.

[hskoglund]

For completeness, here's a copy of some examples from https://en.cppreference.com/w/cpp/language/initialization.html:

#include <string>
 
std::string s1;           // default-initialization
std::string s2();         // NOT an initialization!
                          // actually declares a function “s2”
                          // with no parameter and returns std::string
std::string s3 = "hello"; // copy-initialization
std::string s4("hello");  // direct-initialization
std::string s5{'a'};      // list-initialization (since C++11)
 
char a[3] = {'a', 'b'}; // aggregate initialization
                        // (part of list initialization since C++11)
char& c = a[0];         // reference initialization

So indeed there's a big difference between fn; and fn(); :-)

[Kent-Dorfman]

In the C++ world don't pass mutable objects by const value. Pass them by const reference. ie

const QString& filename

QFile follows this paradigm...