Metaprogramming in C++: Dynamic Build Based on Customized Logics
Metaprogramming in C++ allows developers to write code that can manipulate, generate, or modify other code at compile-time. In this context, we'll explore popular frameworks for metaprogramming in C++ that enable dynamic build based on customized logics.
Introduction to the Problem
Let's consider a scenario where we have a bunch of computational nodes or functions in our source code, such as:
double sum(...) { ... }
double prod(...) { ... }
double minus(...) { ... }
double div(...) { ... }We want to parse a configuration file (cfg) in plaintext format, which describes the logic for a dynamic function implementation. For example:
sum(minus(5, 6), minus(3, 4))This cfg should implicitly create a new function (final_func) in the source code, compile the binary with only the necessary functions, and exclude unnecessary code.
Naive Approach
One naive approach to solve this problem is to:
1. Write a parsing application that generates the source code for final_func based on the cfg.
2. Set flags and use macros to include only the relevant functions.
3. Save the modified source code to a temporary location.
4. Use a bash script to invoke the compiler on the generated source code to build the final binary.
Better Approaches
Experts suggest alternative approaches to achieve this goal more structurally:
• JIT (Just-In-Time) compilation: Implement a parsing function that prepares a Directed Acyclic Graph (DAG) on the fly in the final binary, compiling with all available nodes/functions.
• Embedding a scripting language: Use a scripting language that calls your computational nodes.
• Metaprogramming with frameworks: Utilize popular metaprogramming frameworks in C++, such as:
• Boost: Specifically, Boost.dll can be used to dynamically load final_func after compilation.
• LLVM: Skip bash scripting by leveraging LLVM to generate machine code at compile-time.
• Pre-build steps: Create a pre-build step in your build system to parse the cfg and generate the necessary code.
Key Insights
• Dead code removal: The linker should automatically remove unreferenced code, so you typically don't need to mark functions for exclusion.
• ** Parsing and inference**: If your config is not 1-1 valid C++ code, you may need a two-step process: parsing the cfg to generate valid code, and then including this generated file to compile.
Quotes from Experts
• "You could use something like boost.dll and dynamically load final_func you just compiled. And for that I think you could look into llvm to skip bash." - kpt_ageus
• "The main gist I am getting out of this is that this cannot be done without writing and compiling a parsing app to be used in the pre-build step, so 2 compilations: one for the parsing app, and one for the final binary after parsing app has run." - MindsAndMachines
By exploring these alternative approaches and leveraging the power of metaprogramming in C++, you can create more efficient and dynamic build processes tailored to your specific needs.