How to avoid Clazy warning "Allocating an unneeded temporary container" when joining keys?

Jens G

For some message I need a list of QMap keys as a string. How can following (simplified) code be improved to avoid said Clazy warning?

const QMap<QString, int> shoppingList {{"Bananas", 5}, {"Bread", 1}, {"Milk", 2}};
QString keysText = shoppingList.keys().join(", ");

JonB

@Jens-G
I think it does not like you generating keys() and then iterating it to join(). I think it would like you to write something like:

for (const auto & element : shoppingList )
    keysText += ...

or use QMap::key_iterator QMap::keyBegin() const or similar iterator.

Though how that is as convenient as being able to use join(", ") I don't know/see! Unless an expert knows better. Sounds to me like you/we may know better than clazy here :) [Oh, unless you are using a million+ keys, then this is good advice :) ]

DerReisende

@Jens-G As @JonB already said clazy complains about the temporary QList returned by .keys().
To prevent it you could either use QMapIterator or QMap::key_iterator to join your code.
I made some quick and dirty "performance" measurements on my machine and STL-style key iterators seem to be consistently faster than the other methods:

.join():
T1:  "Bananas, Bread, Milk"
Timing:  3900  nanoseconds.
QMapIterator:
T2:  "Bananas, Bread, Milk"
Timing:  4200  nanoseconds.
STL-style:
T3:  "Bananas, Bread, Milk"
Timing:  1500  nanoseconds.

Done on Windows 11, Visual C++ 2022, Qt 6.5.0 Release mode with AVX2