Understanding Type Erasure: Solving Generic Programming Challenges in C++

08 Jun 2025

Generic programming in C++ often requires carrying type information through function signatures or data structures. This can complicate interfaces and usage when the exact types are not known at compile time or when they vary. One common challenge is how to write code that can accept and operate on heterogeneous types without exposing those types explicitly to every caller.

This blog post explains the problem and demonstrates how type erasure can help manage this complexity, allowing generic values to be stored and passed around without the caller needing to know their concrete types.

The Problem: Carrying Types Through Callers

Consider a templated class or function that operates on a generic type T:

template <typename T>
void process(T value) {
    // do something with value
}

Every caller of process needs to know the type T at compile time. If you want to store or pass these values through interfaces or containers without exposing T, you end up needing to carry that type information throughout your codebase. This complicates the API and often leads to heavy use of templates, which increases compile times and code bloat.

The question arises: Is there a way to store and operate on generic values without exposing their types to the caller?

Type Erasure: Hiding Types Behind a Uniform Interface

Type erasure is a technique that hides the concrete type of an object behind an abstract interface. This allows you to store and manipulate objects of different types through a common API, without the callers needing to know the actual types involved.

The basic idea involves:

• Defining an abstract interface (a concept) with virtual functions.
• Implementing concrete models templated on the actual types.
• Storing pointers to the abstract interface, erasing the concrete type information.

Example: TypeErased Wrapper

Below is an example demonstrating type erasure in C++:

#include <iostream>
#include <memory>

namespace typeerasure {

struct TypeErased final {
    template <typename T>
    TypeErased(T value)
        : m_self(std::make_unique<Model<T>>(std::move(value))) {};
    void print() const {
        m_self->print();
    }
private:
    struct Concept {
        virtual void print() const = 0;
        virtual ~Concept() = default;
    };
    template <typename T>
    struct Model : Concept {
        T value;
        Model(T v) : value(std::move(v)) {}
        void print() const override {
            std::cout << value << '\\n';
        }
    };
    std::unique_ptr<Concept> m_self;
};

void test() {
    // TypeErased does not carry its template parameter around
    TypeErased foo1 = 43;
    TypeErased foo2 = std::string("Hello");
}

// No need for carrying out the generic type parameter
void acceptsFoo(const TypeErased& foo)
{
}

} // namespace typeerasure

Explanation:

• TypeErased is a non-templated class that internally holds a pointer to an abstract Concept.
• The Concept interface defines the operations available (here, print()).
• The Model<T> template implements Concept for any concrete type T.
• When constructing a TypeErased object, a Model<T> is created dynamically with the concrete value.
• The caller only interacts with TypeErased objects through the uniform interface, without needing to know the underlying type.

This design allows code to store and pass generic objects without templates or type parameters appearing in the public API.

Benefits of Type Erasure

• Simplified APIs: Callers use a uniform interface without template parameters.
• Runtime polymorphism: Different types can be stored and handled uniformly.
• Reduced compile-time dependencies: Templates and their instantiations are limited to internal implementation.

Conclusion

Type erasure is a powerful technique for managing generic programming challenges in C++. It lets you abstract away type details behind a common interface, reducing complexity for callers and improving code flexibility.

The example above shows a minimal implementation of type erasure, focusing on a print() operation. This pattern can be extended to other use cases requiring runtime polymorphism without exposing type details.