Careful Handling of Resources

The careful handling of resources - may it be for example memory, files or sockets - is a key concern of programming in C++. This holds in particular true for the embedded programming that is often characterized by limited resources. Therefore, I will write a few posts about this challenging and versatile topic.

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Generalized Plain Old Data

Plain Old Data (POD) obeys the C standard layout. Therefore, you directly can apply the fast C functions memcopy, memmove, memset, or memcmp.

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Tags: enum
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Buckets, Capacity, and Load Factor

The hash function maps a potential infinite number of keys on a finite number of buckets. What is the strategy of the C++ runtime and how can you tailor it to your needs, that is what this article is all about.

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Hash Functions

The reason for the constant access time (best cast) of the unordered associative containers are the hash functions. As ever, C++ offers a lot of ways to adjust the behaviour of the hash functions. On one hand, C++ has a lot of different hash functions; on the other hand, you can define your own hash function. You can even adjust the number of buckets.

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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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Tags: constexpr
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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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Tags: constexpr
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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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Tags: constexpr
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