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# Supplying main() yourself
The easiest way to use Catch is to let it supply ```main()``` for you and handle configuring itself from the command line.
This is achieved by writing ```#define CATCH_CONFIG_MAIN``` before the ```#include "catch.hpp"``` in *exactly one* source file.
Sometimes, though, you need to write your own version of main(). You can do this by writing ```#define CATCH_CONFIG_RUNNER``` instead. Now you are free to write ```main()``` as normal and call into Catch yourself manually.
You now have a lot of flexibility - but here are three recipes to get your started:
## Let Catch take full control of args and config
If you just need to have code that executes before and/ or after Catch this is the simplest option.
```c++
#define CATCH_CONFIG_RUNNER
#include "catch.hpp"
int main( int argc, char* argv[] ) {
// global setup...
int result = Catch::Session().run( argc, argv );
// global clean-up...
return result;
}
```
## Amending the config
If you still want Catch to process the command line, but you want to programatically tweak the config, you can do so in one of two ways:
```c++
#define CATCH_CONFIG_RUNNER
#include "catch.hpp"
int main( int argc, char* argv[] )
{
Catch::Session session; // There must be exactly one instance
// writing to session.configData() here sets defaults
// this is the preferred way to set them
int returnCode = session.applyCommandLine( argc, argv );
if( returnCode != 0 ) // Indicates a command line error
return returnCode;
// writing to session.configData() or session.Config() here
// overrides command line args
// only do this if you know you need to
int numFailed = session.run();
// numFailed is clamped to 255 as some unices only use the lower 8 bits.
// This clamping has already been applied, so just return it here
// You can also do any post run clean-up here
return numFailed;
}
```
Take a look at the definitions of Config and ConfigData to see what you can do with them.
To take full control of the config simply omit the call to ```applyCommandLine()```.
## Adding your own command line options
Catch embeds a powerful command line parser called [Clara](https://github.com/philsquared/Clara).
As of Catch2 (and Clara 1.0) Clara allows you to write _composable_ option and argument parsers,
so extending Catch's own command line options is now easy.
```c++
#define CATCH_CONFIG_RUNNER
#include "catch.hpp"
int main( int argc, char* argv[] )
{
Catch::Session session; // There must be exactly one instance
int height = 0; // Some user variable you want to be able to set
// Build a new parser on top of Catch's
using namespace Catch::clara;
auto cli
= session.cli() // Get Catch's composite command line parser
| Opt( height, "height" ) // bind variable to a new option, with a hint string
["-g"]["--height"] // the option names it will respond to
("how high?"); // description string for the help output
// Now pass the new composite back to Catch so it uses that
session.cli( cli );
// Let Catch (using Clara) parse the command line
int returnCode = session.applyCommandLine( argc, argv );
if( returnCode != 0 ) // Indicates a command line error
return returnCode;
// if set on the command line then 'height' is now set at this point
if( height > 0 )
std::cout << "height: " << height << std::endl;
return session.run();
}
```
See the [Clara documentation](https://github.com/philsquared/Clara/blob/master/README.md) for more details.
## Version detection
Catch provides a triplet of macros providing the header's version,
* `CATCH_VERSION_MAJOR`
* `CATCH_VERSION_MINOR`
* `CATCH_VERSION_PATCH`
these macros expand into a single number, that corresponds to the appropriate
part of the version. As an example, given single header version v2.3.4,
the macros would expand into `2`, `3`, and `4` respectively.
---
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