Associative Containers - A simple Performance Comparison

Before I take a deeper look insight the interface of the hash tables - officially called unordered associative containers - I will at first compare the performance of the associative containers. The best candidates are std::unordered_map and the ordered pendant std::map because both are used most frequently.

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Hash Tables

We missed hash table in C++ for a long time. They promise to have constant access time. C++11 has hash tables in four variations. The official name is unordered associative containers. Unofficially, they are called dictionaries or just simple associative arrays. 

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Type-Traits: Performance Matters

If you look carefully, you see, type-traits have a big optimization potential. The type-traits support in the first step to analyse the code at the compile time and in the second step, to optimize the code based on that analysis. How is that possible? Dependent on the type of a variable a faster variant of an algorithm will be chosen.

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constexpr Functions

constexpr functions are functions that can be executed at compile time. Sounds not so thrilling. But it is. Trust me. You can perform with constexpr functions a lot of calculations at compile time. Therefore, the result of the calculation is at runtime as a constant in ROM available. In addition, constexpr functions are implicit inline.

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constexpr - Variables and Objects

If you declare a variable as constexpr the compiler will evaluate them at compile time. This holds not only true for built-in types but also for instantiations of user-defined types. There are a few serious restrictions for objects in order to evaluate them at compile time.

 

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Constant Expressions with constexpr

You can define with the keyword constexpr an expression that can be evaluated at compile time. constexpr can be used for variables, functions, and user-defined types. An expression that is evaluated at compile time has a lot of advantages. For example constexpr variables and instances of user-defined types are automatically thread-safe and can be stored in ROM; constexpr functions that are evaluated at compile time, are totally done with their work at run time.

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inline

Thanks to inline the compiler can replace the function call by the function body. There are two reasons to use inline functions: performance and safety.

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