CMake's ABSTRACT Property: Enhancing Interoperability with Code Generation and Wrapping Tools

Purpose

• It's primarily intended for tools that wrap classes from your project into other languages. These tools can leverage this information to adjust their wrapping behavior for abstract classes.
• The ABSTRACT property in CMake is used to indicate that a source file defines an abstract class in languages that support this concept (like C++ or Java).

Functionality

• By setting this property on a source file that defines an abstract class, you're essentially providing a hint to these wrapping tools.
• The property serves as a metadata flag that external tools can interpret.
• CMake itself doesn't directly process the ABSTRACT property during the build process.

Common Use Cases

• Code Generation Tools
  Some code generation tools might take the ABSTRACT property into account when generating code for your classes. For example, they might avoid generating certain boilerplate code that wouldn't be applicable to abstract classes.
• Language Binding Tools
  If you're using a tool to generate bindings for your C++ classes in another language (like Python or JavaScript), the ABSTRACT property can help the tool create appropriate bindings for abstract classes. These bindings might be limited or behave differently compared to non-abstract classes.

Setting the Property

set_source_files_properties(MyAbstractClass.cpp PROPERTIES ABSTRACT TRUE)

• The effectiveness of the ABSTRACT property depends on the specific wrapping or code generation tool you're using. Consult the tool's documentation for details on how it handles this property.
• The ABSTRACT property is not a substitute for proper abstract class declarations in your source code. Always ensure your code follows the language's syntax for defining abstract classes.

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Scenario

• We're using a hypothetical wrapping tool called cpp2py to create Python bindings for our classes.
• We have a C++ project with a source file Animal.cpp that defines an abstract class Animal.

CMakeLists.txt

# Define source files
set(SOURCE_FILES Animal.cpp)

# Set ABSTRACT property for Animal.cpp
set_source_files_properties(Animal.cpp PROPERTIES ABSTRACT TRUE)

# Add source files
add_executable(my_animal_cpp ${SOURCE_FILES})

# Hypothetical command for cpp2py (replace with actual tool usage)
# This is for illustrative purposes only!
command(cpp2py WRAP Animal.cpp OUTPUT animal_bindings.py)

1. We define the source file Animal.cpp.
2. We use set_source_files_properties to mark Animal.cpp as containing an abstract class.
3. We add Animal.cpp to our executable my_animal_cpp.
4. The hypothetical cpp2py command (replace with the actual tool's command) wraps Animal.cpp and generates Python bindings in animal_bindings.py.

Animal.cpp

#include <iostream>

class Animal {
public:
  virtual void makeSound() = 0; // Pure virtual function (abstract)
};

class Dog : public Animal {
public:
  void makeSound() override {
    std::cout << "Woof!" << std::endl;
  }
};

int main() {
  // You cannot create an object of an abstract class
  // Animal animal;  // This would cause an error

  Dog dog;
  dog.makeSound(); // Prints "Woof!"
  return 0;
}

• cpp2py might create different bindings for Animal compared to a concrete class like Dog, potentially making Animal unusable from Python directly. Refer to your actual wrapping tool's documentation for specific behavior.
• The ABSTRACT property in CMake informs the hypothetical cpp2py tool that Animal is abstract.
• Animal.cpp defines an abstract class Animal with a pure virtual function makeSound().

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Rely on Language-Specific Mechanisms

• Abstract Class Declarations
  The most effective way to indicate an abstract class is to use the language's built-in mechanisms for defining abstract classes. In C++, this involves declaring pure virtual functions in the base class. In Java, use abstract methods. This ensures proper behavior in your code regardless of external tools.

Code Comments or Naming Conventions

• Comments
  You can add comments within your code to explicitly state that a class is abstract. This might be helpful for human readers but doesn't provide automated processing. For example:

// This is an abstract class
class Animal {
  // ...
};

• Naming Conventions
  Consider using a consistent naming convention for abstract classes (e.g., prefixing them with Abstract or Base). This can help developers easily identify them within the codebase. However, this is less reliable than comments or language-specific methods.

Custom CMake Properties (Limited Use)

• While not ideal, you could create custom properties in CMake to store information about abstract classes. However, this requires manual intervention and isn't as widely supported by external tools. It's generally preferable to rely on language-specific mechanisms or comments.

The best approach depends on your specific needs:

• If you need a temporary or less structured approach, comments or naming conventions could suffice.
• For code clarity and maintainability, language-specific mechanisms are the most reliable solution.
• If you primarily rely on language bindings or code generation tools, ABSTRACT can be helpful if the tool supports it.