Megh's Blog

C++20 Concepts in C++03

C++20 Concepts are a new language feature that ease generic programming, but are primarily syntactic sugar.

We will try to implement them in C++03, with one caveat - we must explicitly specify that a class implements an concept.

NOTE: We will use template specialization and do not need to be able to modify the class or our concept for this.

NOTE: If it seems like the caveat ignores the entire point of concepts, call these "pseudo-minimal-rust-traits" and read on. By the end of the article, as the Zen of Python mentions, I promise you will agree that explicit is better than implicit :P

What are C++ Concepts #

C++ Concepts allow us to do compile-time dispatch of methods.

This compile-time dispatch is thus kind of like Rust traits. (Rust traits provide other features too.)

▶ 
// Define the concept check
template <typename Self>
concept Counter = requires(Self counter, int new_count) {
    { counter.get_count() } -> std::same_as<int>;
    { counter.set_count(new_count) } -> std::same_as<void>;
    { Self::max_count() } -> std::same_as<int>;
};

// Our struct
struct MyCounter {
    int count;

    int get_count() { return count; }
    void set_count(int new_count) { count = new_count; }
    static int max_count() { return 100; }
};
static_assert(Counter<MyCounter>); // optionally check implementation
                                   // if we forgot any methods, etc...

// Example usage
template <typename T>
requires Counter<T>
void print_counter(T& counter) {
    // Can also use shorthand `template <Counter T>` instead of `requires` clause
    std::cout << "Counter with count " << counter.get_count() << std::endl;
}

// compile-time dispatch another method
void print_counter(int counter) {
    std::cout << "Integer counter with count " << counter << std::endl;
}

// Shorthand syntax
void print_counter_shorthand(Counter auto counter) { print_counter(counter); }

int main() {
    MyCounter c { 25 };
    print_counter(c);
    print_counter(10); // Prints Integer counter
    print_counter_shorthand(c);
}

Notice that we never needed to specify that MyCounter implements Counter. This can easily be fixed by requiring some constant to be defined in MyCounter or otherwise.

We call the above "implicit concepts". We will try to implement "explicit concepts" - where something must specify that the concept has been implemented for a class.

C++03 Concepts #

We will use C++11 initially. Then will also modify this using some macros for C++03.

We use a templated struct and observe that static_asserts inside it are executed when the template is specialized.

▶ 
// Define the concept
struct Counter {
    // This template is specialized to true_type by any class that
    // implements this. Specialization requires ownership of neither
    // Counter concept or Self (Self being the class in question)
    template <typename Self>
    struct is_implemented_by: std::false_type {};

    // Define the check
    template <typename Self>
    struct check: std::true_type {
        static_assert(static_cast< int (Self::*)() >(&Self::get_count));
        static_assert(static_cast< void (Self::*)(int) >(&Self::set_count));
        static_assert(static_cast< int (*)() >(&Self::max_count));
    };
};

// Define our struct normally, without any modifications
struct MyCounter {
    int count;

    int get_count() { return count; }
    void set_count(int new_count) { count = new_count; }
    static int max_count() { return 100; }
};
// Declare and check that we have implemented the trait
template<>
struct Counter::is_implemented_by<MyCounter>: Counter::check<MyCounter> {};

// Example usage using classic enable_if SFINAE. Verbose yet conventional
template <typename T>
typename std::enable_if<
    Counter::is_implemented_by<T>::value,
void >::type
print_counter(T& counter) {
    std::cout << "Counter with count " << counter.get_count() << std::endl;
}

// compile-time dispatch another method
void print_counter(int counter) {
    std::cout << "Integer counter with count " << counter << std::endl;
}

int main() {
    MyCounter counter { 25 };
    print_counter(counter);
    print_counter(10); // Prints Integer counter
}

To do this in C++03, and make it work with C++11 too, lets sprinkle some macros.

▶ 
// Define the concept
struct Counter {
    template <typename T>
    struct is_implemented_by: std::false_type {};

    // Define the check
    CONCEPT_CHECK_BEGIN
        CONCEPT_ASSERT(static_cast< int (Self::*)() >(&Self::get_count));
        CONCEPT_ASSERT(static_cast< void (Self::*)(int) >(&Self::set_count));
        CONCEPT_ASSERT(static_cast< int (*)() >(&Self::max_count));
    CONCEPT_CHECK_END
};

// ... skipping definition of struct MyCounter ...
// Declare and check that we have implemented the trait
IMPL_CONCEPT(Counter, MyCounter);

We can now use this for defining print_count using the same enable_if way we used previously. Also most of our macros are simple ones that dont require any parenthesis-escaping except IMPL_CONCEPT. Note these macros are completely optional in C++11.

C++03 details and the macros

BOOST_STATIC_ASSERT can not take reference to a function (and static_assert is C++11)

<source>:67:67: error: '&' cannot appear in a constant-expression
   67 |         BOOST_STATIC_ASSERT(static_cast< int (Self::*)() >(&Self::get_count));

We get around this by defining the CONCEPT_ASSERT macro expands an empty function in C++03, and the CONCEPT_CHECK_BEGIN defines the constructor of a check<Self> struct. This object is then internal-linkage-constructed by IMPL_CONCEPT. This ensures that the compiler tries to specialize the constructor with Self and detects that the static_casts failed.

Note the CONCEPT_ASSERT macro should not be used for "normal"/non-method check asserts as it simply does nothing. Use say BOOST_STATIC_ASSERT otherwise.

See example preprocessor output here

We can check the compile time error because get_count is commented out

Aside: Explicit concepts in C++20 #

Explicit concepts can be implemented in pretty much the same way in C++20, using a templated is_counter conditional struct

▶ 
// Explicit check
template <typename Self>
struct is_counter: std::false_type {};

// Define the concept check
template <typename Self>
concept Counter =
    is_counter<Self>::value &&               // ** NEW **
    requires(Self counter, int new_count) {  // ** SAME STUFF **
        { counter.get_count() } -> std::same_as<int>;
        { counter.set_count(new_count) } -> std::same_as<void>;
        { Self::max_count() } -> std::same_as<int>;
    };

// ... skipping definition of struct MyCounter ...
// Declare and check that we have implemented the trait
template <> struct is_counter<MyCounter> : std::true_type {};
static_assert(Counter<MyCounter>); // optionally check implementation
                                   // if we forgot any methods, etc...

Rant on C++20 Concepts #

C++20 Concepts thus allow for powerful implicit matching. But, let us take the following example:

Lets say we are building some kind of social media stats app and we have youtube_api::VideoViewCounter and instagram_api::LikeCounter. Both of them have the get_count method.

We want to define a print_count(counter) method which takes either of these two classes and does std::cout << counter.get_count().

We do not have control over either APIs, but would like a common abstraction. We can:

  1. Declare an "implicit concept" called Counter which requires a get_count method. Define templated print_count for concept
  2. Declare an "explicit concept" with the same. Specify that the above two classes implement this concept without modying the classes. Define templated print_count for concept.
  3. Use an unchecked templated print_count

Now consider the following modification to the codebase:

We add class my_shared_ptr which has get_count method which returns the reference count of the pointer.

Lets say another engineer started refactoring to store the objects in shared_ptr but print_counter has not been modified for an explicit overload for shared_ptr

What happens when we call print_count(counter_ptr) with counter_ptr = my_shared_ptr<youtube_api::VideoViewCounter>()?

Note that:

  1. In the case of "implicit concepts", we would see the reference count being printed, without any compile or run-time error.
  2. In the case of "explicit concepts", we would get a compile time error since no method matches this.
  3. In the case of an unchecked template too, we would see the reference count being printed too.

Thus, implicit concepts are almost as bad as not having any check at all. Except maybe they can produce neater compiler errors (ignoring the case of overloading based on concepts).

Even if you had a 1000 different classes, writing 1000 more lines saying that a concept is implemented by them is better than implicit behaviour in my opinion. In most cases you will either have 1000 template specializations or some script generated code, and in both cases you only need to add one line.

What regex is to parsing, implicit concepts are to C++.

And if explicit concepts are better and already implementable in C++03, why provide an abstraction where most developers will write error-prone code instead of providing syntax sugar for explicit concepts?

Extra syntactic sugar stuff #

Summary #

We saw what C++ concepts were and how to write "explicit concepts" in C++03. We also noted that C++20 implicit concepts are error-prone.

In the next post I will describe how this compares to Rust traits and how to implement "trait objects" or "concept maps" using the same code.

NOTE: For any of the "predefined concepts" like say copy_constructible, type_traits or similar Boost/utility library can be used.