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C++ Type Traits & static_assert: Defensive Compile-Time Metaprogramming

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C++ Type Traits & static_assert: Defensive Compile-Time Metaprogramming
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C++ Type Traits & static_assert: Defensive Compile-Time Metaprogramming

Table of Contents

Type Traits Categories Overview

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Primary Type Categories: What Kind of Type Is This?


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Type Property Traits


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Supported Operations Traits


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Type Transformation Traits


Transformation traits produce a new type — used in template helpers and wrappers:


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std::conditional: Type-Level if/else


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Writing Custom Type Traits


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Detection Idiom: Detecting Member Functions


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static_assert: ABI and Layout Validation


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Frequently Asked Questions


When should I use type traits vs Concepts?
Use Concepts in function/class template parameter lists for constraints (better error messages). Use type traits inside function bodies with if constexpr for compile-time branching (generate different code for different types). Use static_assert + type traits for ABI and layout guarantees that must be enforced unconditionally. All three work together — they're not mutually exclusive.


What is void_t and why does it work?
std::void_t<...> maps any number of valid types to void. In a class template partial specialization, if any of the types in void_t<...> fail to form (because an expression is invalid), SFINAE kicks in and the specialization is discarded. This is how the detection idiom works — the decltype(...) expression fails if the member function doesn't exist, discarding the true_type specialization.


Are type traits available in constexpr context?
Yes — all _v variable templates (std::is_integral_v<T>, etc.) are constexpr bool and can be used in if constexpr, static_assert, template argument positions, and any compile-time context. They're computed entirely at compile time.




Key Takeaway


Type traits are the introspection toolkit that makes C++ generic code safe and precise. They answer "what is this type?" at compile time, enabling code that branches, validates, and transforms based on type properties — entirely without runtime overhead. Combined with Concepts (which use the same type-checking machinery under the hood), if constexpr for dispatch, and static_assert for validation, type traits are the scaffolding of every professional C++ library.


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