README-qstring-allocations.md

qstring-unneeded-heap-allocations

Finds places with unneeded memory allocations due to temporary QStrings.

Here's a summary of usages that allocate:

1. QString s = "foo"; // Allocates, use QStringLiteral("foo") instead

2. QString s = QLatin1String("foo"); // Allocates, use QStringLiteral("foo") instead

   2.1 QString s = QLatin1String(""); // No allocation. QString is optimized for this case, so it's safe for empty literals

3. QString s = QStringLiteral("foo"); // No allocation

4. QString s = QString::fromLatin1("foo"); // Allocates, use QStringLiteral

5. QString s = QString::fromUtf8("foo"); // Allocates, use QStringLiteral

6. s == "foo" // Allocates, use QLatin1String

7. s == QLatin1String("foo) // No allocation

8. s == QStringLiteral("foo") // No allocation

9. QString {"append", "compare", "endsWith", "startsWith", "indexOf", "insert", "lastIndexOf", "prepend", "replace", "contains" } // They all have QLatin1String overloads, so passing a QLatin1String is ok.

10. QString::fromLatin1("foo %1").arg(bar) // Allocates twice, replacing with QStringLiteral makes it allocate only once.

Fixits

fix-qlatin1string-allocations        // To replace QLatin1String with QStringLiteral only where it was allocating before
fix-fromLatin1_fromUtf8-allocations  // To replace fromLatin1() and fromUtf8() so it doesn't allocate
fix-fromCharPtrAllocations           // To replace raw string literals so it doesn't allocate

Example:
    export CLAZY_FIXIT="fix-fromCharPtrAllocations"

Pitfalls

• QStringLiteral might make your app crash at exit if plugins are involved. See: https://blogs.kde.org/2015/11/05/qregexp-qstringliteral-crash-exit and http://lists.qt-project.org/pipermail/development/2015-November/023681.html

• Also note that MSVC crashes when QStringLiteral is used inside initializer lists. For that reason no warning or fixit is emitted for this case unless you set an env variable:

    export CLAZY_EXTRA_OPTIONS="qstring-allocations-no-msvc-compat"