---
title: "Obscure Qt: Testing log messages"
date: 2025-04-15
draft: false
tags:
- Qt
series:
- Obscure Qt
summary: "I noticed that I kept searching for \"how do I check for log messages in Qt tests?\" online. I figure that is a good enough indicator for \"obscure\" and now deserves to be saved on my blog"
---

I noticed that I kept searching for "how do I check for log messages in Qt tests?" online. I figure that is a good enough indicator for obscurity[^1] and now deserves to be saved on my blog!

The case that spurred this is me writing a bunch of fallible code recently, and I wanted to check if the log message is emitted. Since the message also dynamic, making sure it has the correct line number too. For reference, here is some sample code:

```cpp
const auto node = childNode.firstChildElement(QStringLiteral("SomeNode"));
if (node.isNull()) {
    qWarning() << "Missing <SomeNode> for node at line" << childNode.lineNumber();
    return std::nullopt;
}
...
```

Checking if this returns `std::nullopt` is easy enough, but what about the log output? It turns out QTest has us covered here with [QTest::ignoreMessage](https://doc.qt.io/qt-6/qtest.html#ignoreMessage). Add it to your test case, and it will panic if the message is not emitted:

```cpp
QTest::ignoreMessage(QtWarningMsg, "Missing <SomeNode> for node at line 14");
```

And if the message isn't emitted, it will complain in your test output:

```shell
INFO   : ConfigFileTest::scaleUpData() Did not receive message: "Missing <SomeNode> for node at line 14"
FAIL!  : ConfigFileTest::scaleUpData() Not all expected messages were received
```

However if the message is found, it _hides said message from the test output_ which is extremely useful too. Also useful to know is that there's [a RegExp overload](https://doc.qt.io/qt-6/qtest.html#ignoreMessage-1) if you don't want it to be as specific.

[^1]: Out of pure curiosity, I checked the KDE repositories and there's only ~60 usages of this function. I hope to see more of this the next time I check!