Another frequently asked question in C++ newsgroups: “Is there a tool that decompiles an executable program back into C++ source code?” The short answer is “no, there isn’t”. The longer answer will also convince you why trying to find such a tool is at least as futile as trying to find the Holy Grail.

First, remember that there is no 1:1 relation between a piece of assembly code and a corresponding source code written in a high-level language such as C++. For example, for loops, switch statements, do, and while blocks are all translated into the same assembly directives. In other words, there is an infinite number of C++ programs that can produce the exact assembly code. The same is true for other language constructs, which are translated into a single assembly entity: pointers, references, and arrays for example.

Furthermore, each compiler produces a different executable for a given source file. Even if you know the exact compiler brand that was used for compiling the program, as well as its version number, how can you tell which compile-time options were used during compilation? Also, how can a decompiler reconstitute macros, inline functions, and typedef’s when all these are all substituted before the compiler actually translates the source code into machine code? Remember also that normally, the debugging information is removed from the executable so the decompiler will not be able to reconstitute the original names of variables, classes, arrays, pointers, functions, and constants. To conclude, developing a utility that decompiles an executable file into its original C++ source code (or anything that comes close to the original) is unrealistic.