I presume that by "C++ code" you mean something that mutates the source code itself rather than a compiled version? Source code mutation is far harder to implement than intermediate code mutation (e.g. Java bytecode or .NET IL). Because of this, I strongly suspect that you won't find an open source one.
The challenge is to parse the source code into some form of syntax tree, a hard problem in C++, which will then allow you to identify mutation points and make the source code changes you need. You might like to take a look at GCCXML as an open source starting point for parsing - adding the mutation is actually the more straightforward part of the problem.
The open source NinjaTurtles (disclaimer: I am lead developer on this) will mutate assemblies compiled from .NET managed C++ code, but I suspect that won't be any good to you?