From a73385e2cfd0455f09166cdb380876ba8b26fb5f Mon Sep 17 00:00:00 2001
From: Marc Mutz <marc.mutz@kdab.com>
Date: Mon, 13 May 2019 20:43:27 +0200
Subject: [PATCH] Short live qExchange()!

This is a 1:1 replacement for std::exchange, and should be removed once
Qt fully depends on C++14. It's too versatile a tool to miss it, so
provide a copy.

[ChangeLog][QtCore][QtGlobal] Added qExchange(), a drop-in for C++14's std::exchange()

Change-Id: I31c4f1141e7a99f99ea65eb36ddf9d68b7847337
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 src/corelib/global/qglobal.cpp | 11 +++++++++++
 src/corelib/global/qglobal.h   |  9 +++++++++
 2 files changed, 20 insertions(+)

diff --git a/src/corelib/global/qglobal.cpp b/src/corelib/global/qglobal.cpp
index 88a61562cb..3436a19881 100644
--- a/src/corelib/global/qglobal.cpp
+++ b/src/corelib/global/qglobal.cpp
@@ -3805,6 +3805,17 @@ bool qunsetenv(const char *varName)
     \snippet code/src_corelib_global_qglobal.cpp as-const-4
 */
 
+/*!
+    \fn template <typename T, typename U = T> T qExchange(T &obj, U &&newValue)
+    \relates <QtGlobal>
+    \since 5.14
+
+    Replaces the value of \a obj with \a newValue and returns the old value of \a obj.
+
+    This is Qt's implementation of std::exchange(). It differs from std::exchange()
+    only in that it is \c constexpr already in C++14, and available on all supported
+    compilers.
+*/
 /*!
     \macro QT_TR_NOOP(sourceText)
     \relates <QtGlobal>
diff --git a/src/corelib/global/qglobal.h b/src/corelib/global/qglobal.h
index 0a0c434a07..24d250d923 100644
--- a/src/corelib/global/qglobal.h
+++ b/src/corelib/global/qglobal.h
@@ -1008,6 +1008,15 @@ Q_DECL_CONSTEXPR typename std::add_const<T>::type &qAsConst(T &t) noexcept { ret
 template <typename T>
 void qAsConst(const T &&) = delete;
 
+// like std::exchange
+template <typename T, typename U = T>
+Q_DECL_RELAXED_CONSTEXPR T qExchange(T &t, U &&newValue)
+{
+    T old = std::move(t);
+    t = std::forward<U>(newValue);
+    return old;
+}
+
 #ifndef QT_NO_FOREACH
 
 namespace QtPrivate {