What would be most efficient way to compare QString and char*

Question

if( mystring == mycharstar ) {} will perform malloc,

and

if(strcmp(mystring.toLocal8Bit().constData(),mycharstar ) == 0) {}

Will allocate a QByteArray

I would like to not have any allocation happening, would would you guys recommend?

What about

if(mystring == QLatin1String(mycharstar))

Would it be any better?

Instinctively I would do `mystring.toAscii() == mycharstar`, but I haven't profiled, so don't listen to me. — tux3, Mar 10 '15 at 15:30
tux3 - to Ascii will allocate also and will do a conversion. — Kamil Klimek, Mar 10 '15 at 15:52
All depends what encoding you want to use. You will have to make your own compare function probably — Kamil Klimek, Mar 10 '15 at 15:54

score 5 · Answer 1 · edited May 23 '17 at 11:56

There is no "efficient" way that only uses casts. This is because QtString internally uses 16 bits to encode a single character while C strings use only 8 bits. That means any comparison based on memory pointers will simply almost always return false.

That's why you have to encode the 16 bit wide characters of QtString to the same encoding as your C string and that always needs at least one call to malloc().

See also: How to convert QString to std::string?

score 2 · Answer 2 · answered May 15 '18 at 09:51

2

It could be if( mystring == QLatin1String(mycharstar) ), as suggested here.

answered May 15 '18 at 09:51

Michał Leon

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What would be most efficient way to compare QString and char*

2 Answers2