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C/C++ API Documentation | 2026.1

# ClientUser::Merge( FileSys *, FileSys *, FileSys *, FileSys *, Error * )

Call an external merge program to merge three files during resolve.

Virtual? | Yes |
Class | `ClientUser` |
Arguments | `FileSys *base` | the "base" file
| `FileSys *leg1` | the "theirs" file
| `FileSys *leg2` | the "yours" file
| `FileSys *result` | the final output file
| `Error *e` | an `Error` object
Returns | `void` |

## Notes

`Merge()` is called if the “m” option is selected during an interactive resolve. `Merge()` does not call the merge program; it merely invokes external merge programs (including P4 Merge as well as third-party tools). External merge programs must be specified by an environment variable, either `P4MERGE` or `MERGE`. `Merge()` returns after the external merge program exits.

As in `Diff()`, the external program is invoked using `ClientUser::RunCmd()`.

## See also

`ClientUser::RunCmd()`

## Example

When the "merge" option is selected during an interactive resolve, the file arguments to `Merge()` are as follows:

Argument | Value
--- | ---
`base` | A temp file built from the depot revision that is the "base" of the resolve.
`leg1` | A temp file built from the depot revision that is the "theirs" of the resolve.
`leg2` | The local workspace file that is the "yours" of the resolve.
`result` | A temp file in which to construct the new revision of "yours".

These file arguments correspond exactly to the command-line arguments passed to the merge tool.

After you "accept" the merged file (with “ae”), the "result" temp file is copied into the “leg2” or "yours" workspace file, and this is the file that is submitted to the depot.

To change the way that external merge programs are called during a resolve, create a subclass of `ClientUser` with an alternate implementation of `Merge()`.

For example, suppose that one of your favorite merge tools, “yourmerge”, requires the “result” file as the first argument. Rather than wrapping the call to the merge tool in a script and requiring your users to set `P4MERGE` to point to the script, you might want to provide support for this tool from within your application as follows:

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    void MyClientUser::Merge(
        FileSys *base,
        FileSys *leg1,
        FileSys *leg2,
        FileSys *result,
        Error *e )
    {
        char *merger;

        if ( !( merger = enviro-&gt;Get( "P4MERGE" ) ) &amp;&amp;
             !( merger = getenv( "MERGE" ) ) )
        {
            e-&gt;Set( ErrClient::NoMerger );
            return;
        }

        if ( strcmp( merger, "yourmerge" ) == 0 )
        {
            RunCmd( merger, result-&gt;Name(), base-&gt;Name(),
                    leg1-&gt;Name(), leg2-&gt;Name(), 0, e );
        }
        else
        {
            RunCmd( merger, base-&gt;Name(), leg1-&gt;Name(),
                    leg2-&gt;Name(), result-&gt;Name(), 0, e );
        }
    }

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