docs/markdown/Precompiled-headers.md

---
short-description: Using precompiled headers to reduce compilation time
...

# Precompiled headers

Parsing header files of system libraries is surprisingly expensive. A
typical source file has less than one thousand lines of code. In
contrast the headers of large libraries can be tens of thousands of
lines. This is especially problematic with C++, where header-only
libraries are common and they may contain extremely complex code. This
makes them slow to compile.

Precompiled headers are a tool to mitigate this issue. Basically what
they do is parse the headers and then serialize the compiler's
internal state to disk. The downside of precompiled headers is that
they are tricky to set up. Meson has native support for precompiled
headers, but using them takes a little work.

A precompiled header file is relatively simple. It is a header file
that contains `#include` directives for the system headers to
precompile. Here is a C++ example.

```cpp
    #include<vector>
    #include<string>
    #include<map>
```

In Meson, precompiled header files are always per-target. That is, the
given precompiled header is used when compiling every single file in
the target. Due to limitations of the underlying compilers, this
header file must not be in the same subdirectory as any of the source
files. It is strongly recommended that you create a subdirectory
called `pch` in the target directory and put the header files (and
nothing else) there.

Toggling the usage of precompiled headers
--

If you wish to compile your project without precompiled headers, you
can change the value of the pch option by passing `-Db_pch=false`
argument to Meson at configure time or later with `meson configure`. You can
also toggle the use of pch in a configured build directory with the
GUI tool. You don't have to do any changes to the source
code. Typically this is done to test whether your project compiles
cleanly without pch (that is, checking that its #includes are in
order) and working around compiler bugs.

Using precompiled headers with GCC and derivatives
--

Once you have a file to precompile, you can enable the use of pch for
a given target with a *pch* keyword argument. As an example, let's assume
you want to build a small C binary with precompiled headers.
Let's say the source files of the binary use the system headers `stdio.h`
and `string.h`. Then you create a header file `pch/myexe_pch.h` with this
content:

```c
#include <stdio.h>
#include <string.h>
```

And add this to meson:

```meson
executable('myexe', sources : sourcelist, c_pch : 'pch/myexe_pch.h')
```

That's all. You should note that your source files must _not_ include
the file `myexe_pch.h` and you must _not_ add the pch subdirectory to
your search path. Any modification of the original program files is
not necessary. Meson will make the compiler include the pch with
compiler options. If you want to disable pch (because of, say,
compiler bugs), it can be done entirely on the build system side with
no changes to source code.

You can use precompiled headers on any build target. If your target
has multiple languages, you can specify multiple pch files like this.

```meson
executable('multilang', sources : srclist,
           c_pch : 'pch/c_pch.h', cpp_pch : 'pch/cpp_pch.h')
```

Using precompiled headers with MSVC
--
Since Meson version 0.50.0, precompiled headers with MSVC work just like
with GCC. Meson will automatically create the matching pch implementation
file for you.

Before version 0.50.0, in addition to the header file, Meson
also requires a corresponding source file. If your header is called
`foo_pch.h`, the corresponding source file is usually called
`foo_pch.cpp` and it resides in the same `pch` subdirectory as the
header. Its contents are this:

```cpp
#if !defined(_MSC_VER)
#error "This file is only for use with MSVC."
#endif

#include "foo_pch.h"
```

To enable pch, simply list both files in the target definition:

```meson
executable('myexe', sources : srclist,
           cpp_pch : ['pch/foo_pch.h', 'pch/foo_pch.cpp'])
```

This form will work with both GCC and msvc, because Meson knows that
GCC does not need a `.cpp` file and thus just ignores it.

It should be noted that due to implementation details of the MSVC
compiler, having precompiled headers for multiple languages in the
same target is not guaranteed to work.