意外的行为字符串文字

Question

I have two methods:

ind addFilter(ind column, const QString & condition);
ind addFilter(ind column, bool condition);

When I use this construction gcc choose second method (with bool condition )

filters->addFilter(familyNameInd, "М*");

Why std::string convert to bool instead of QString? Or can I specify all string literals to be QString at compilation?

Answer 1

According to the C++ Standard (13.3.3.2 Ranking implicit conversion sequences)

2 When comparing the basic forms of implicit conversion sequences (as defined in 13.3.3.1) — a standard conversion sequence (13.3.3.1.1) is a better conversion sequence than a user-defined conversion sequence or an ellipsis conversion sequence, and ...

Relative to your example converting to bool is a standard implicit conversion. So it s better than user-defined conversion using a conversion constructor. So if you want that the overloaded function with parameter of type const QString & would be called you have to specify explicitly the conversion from the string literal to a temporary object of type QString: QString( "М*" )`.

Answer 2

"M*" is a const char* which is convertible to a bool due to implicit conversion rules. The implicit conversion will be chosen over the QString class.

You are going to have to call the function like this:

filters->addFilter(familyNameInd, QString("М*"));

To avoid an unnecessary copy of the QString, consider using the QStringLiteral macro as well:

filters->addFilter(familyNameInd, QStringLiteral("М*"));

Answer 3

First of all, your code is broken. There is no such an argument and return type as ind . I will assume that you meant int .

Secondly, POD types have precedence over custom types in C++ when it comes to implicit conversion.

Why std::string convert to bool instead of QString?

I do not know why you think it would be std::string . It simply is not, not even in C++. It is the good old const char[X] where X happens to be three in your case (two letters + terminating nil).

Or can I specify all string literals to be QString at compilation?

No, but you ought to use this in any case when dealing with string literals in Qt regardless of this situation:

filters->addFilter(familyNameInd, QStringLiteral("М*"));

For raw string literals like this, please do not use the QString contructor. It is pointless. Therefore, you would be writing something like this:

main.cpp

#include <QString>
#include <QDebug>

int addFilter(int column, const QString & condition) { qDebug() << "Test 1"; }
int addFilter(int column, bool condition) { qDebug() << "Test 2"; }

int main()
{
    addFilter(0, QStringLiteral("foo"));
    return 0;
}

main.pro

TEMPLATE = app
TARGET = main
QT = core
SOURCES += main.cpp

Build and Run

qmake && make && ./main

Output

Test 1