An open source license

The advantage of open source licenses is that we don't have to pay anyone to use Qt; however, the downside is that there are some limitations imposed on how it can be used.

When choosing the open source edition, we have to choose between GPL 3.0 and LGPL 3. Since LGPL is more liberal, in this chapter we will focus on it. Choosing LGPL allows you to use Qt to implement systems that are either open source or closed source—you don't have to reveal the sources of your application to anyone if you don't want to.

However, there are a number of restrictions you need to be aware of:

• Any modifications that you make to Qt itself need to be made public, for example, by distributing source code patches alongside your application binary.
• LGPL requires that users of your application must be able to replace Qt libraries that you provide them with other libraries with the same functionality (for example, a different version of Qt). This usually means that you have to dynamically link your application against Qt so that the user can simply replace Qt libraries with his own. You should be aware that such substitutions can decrease the security of your system; thus, if you need it to be very secure, open source may not be the option for you.
• LGPL is incompatible with a number of licenses, especially proprietary ones, so it is possible that you won't be able to use Qt with some commercial components.

Some Qt modules may have different licensing restrictions. For example, Qt Charts, Qt Data Visualization, and Qt Virtual Keyboard modules are not available under LGPL and can only be used under GPL or the commercial license.

The open source edition of Qt can be downloaded directly from https://www.qt.io.