Improving the qcolor-from-literal Clazy check

January 20, 2024

Improving the qcolor-from-literal Clazy check

For all of you who don't know, Clazy is a clang compiler plugin that adds checks for Qt semantics. I have it as my default compiler, because it gives me useful hints when writing or working with preexisting code. Recently, I decided to give working on the project a try! One bigger contribution of mine was to the qcolor-from-literal check, which is a performance optimization. A QColor object has different constructors, this check is about the string constructor. It may accept standardized colors like “lightblue”, but also color patterns. Those can have different formats, but all provide an RGB value and optionally transparency. Having Qt parse this as a string causes performance overhead compared to alternatives.

Fixits for RGB/RGBA patterns

When using a color pattern like “#123” or “#112233”, you may simply replace the string parameter with an integer providing the same value. Rather than getting a generic warning about using this other constructor, a more specific warning with a replacement text (called fixit) is emitted.

testf4ile.cpp:92:16: warning: The QColor ctor taking RGB int value is cheaper than one taking string literals [-Wclazy-qcolor-from-literal]
        QColor("#123");
               ^~~~~~
               0x112233
testfile.cpp:93:16: warning: The QColor ctor taking RGB int value is cheaper than one taking string literals [-Wclazy-qcolor-from-lite
        QColor("#112233");
               ^~~~~~~~~
               0x112233

In case a transparency parameter is specified, the fixit and message are adjusted:

testfile.cpp:92:16: warning: The QColor ctor taking ints is cheaper than one taking string literals [-Wclazy-qcolor-from-literal]
        QColor("#9931363b");
               ^~~~~~~~~~~
               0x31, 0x36, 0x3b, 0x99

Warnings for invalid color patterns

Next to providing fixits for more optimized code, the check now verifies that the provided pattern is valid regarding the length and contained characters. Without this addition, an invalid pattern would be silently ignored or cause an improper fixit to be suggested.

.../qcolor-from-literal/main.cpp:21:28: warning: Pattern length does not match any supported one by QColor, check the documentation [-Wclazy-qcolor-from-literal]
    QColor invalidPattern1("#0000011112222");
                           ^
.../qcolor-from-literal/main.cpp:22:28: warning: QColor pattern may only contain hexadecimal digits [-Wclazy-qcolor-from-literal]
    QColor invalidPattern2("#G00011112222");

Fixing a misleading warning for more precise patterns

In case a “#RRRGGGBBB” or “#RRRRGGGGBBBB” pattern is used, the message would previously suggest using the constructor taking ints. This would however result in an invalid QColor, because the range from 0-255 is exceeded. QRgba64 should be used instead, which provides higher precision.

I hope you find this new or rather improved feature of Clazy useful! I utilized the fixits in Kirigami, see https://invent.kde.org/frameworks/kirigami/-/commit/8e4a5fb30cc014cfc7abd9c58bf3b5f27f468168. Doing the change manually in Kirigami would have been way faster, but less fun. Also, we wouldn't have ended up with better tooling :)