1. Overview
Function adaptors are a feature of the C++ standard library that allow for the modification of function objects, creating new functions that, for example, take fewer arguments or modify their arguments.
Since the introduction of C++14, function adaptors have often been replaced with lambdas in many coding practices due to their increased flexibility and conciseness. However, function adaptors still hold their ground in cases where the use of lambdas would result in overly complicated or less readable code. In essence, while lambdas offer powerful and succinct solutions, function adaptors can provide a more structured approach, particularly beneficial in larger, more complex software systems.
2. std::bind
std::bind is a standard function adaptor that generates a callable object. It takes a function and binds its one or more arguments. The remaining unbound parameters can be passed in at the call site. It is often used to adapt a function that takes multiple arguments into a function object that takes fewer arguments. Placeholders (from the std::placeholders namespace) can be used to specify arguments that should be supplied later.
Example:
auto bound_fn = std::bind(some_function, 1, std::placeholders::_1); bound_fn(2); // Calls some_function(1, 2)
In this example, some_function is a function that takes two arguments. std::bind is used to bind the first argument to 1, and the second argument is left as a placeholder, meaning it should be supplied when the bound function is called.
3. std::bind_front
std::bind_front is a function adaptor introduced in C++20. It generates a callable object by binding its given arguments to the corresponding parameters of a function, starting from the first parameter. The remaining parameters of the function can be supplied later when calling the returned function object.
Example:
auto bound_fn = std::bind_front(some_function, 1); bound_fn(2, "Test"); // Calls some_function(1, 2, "Test")
In this example, some_function is a function that takes three arguments. std::bind_front is used to bind the first argument to 1, and the remaining arguments are to be supplied when the bound function is called.
4. std::bind_back
std::bind_back is a function adaptor since C++ 23. It is expected to generate a callable object by binding its given arguments to the corresponding parameters of a function, starting from the last parameter. The remaining parameters of the function can be supplied later when calling the returned function object.
Example:
auto bound_fn = std::bind_back(some_function, "Test"); bound_fn(1, 2); // Calls some_function(1, 2, "Test")
In this example, assuming some_function is a function that takes three arguments, std::bind_back is used to bind the last argument to "Test", and the remaining arguments are to be supplied when the bound function is called.
5. Conclusions
These function adaptors provide a way to customize the behavior of functions and create more flexible and reusable function objects. They allow for function composition and can be particularly useful in conjunction with other C++ features such as lambda expressions and algorithms.