p0919R1 Heterogeneous lookup for unordered containers (C++20 proposal paper) by Mateusz Pusz.Heterogeneous Lookup in Ordered Containers, C++14 Feature by Bartlomiej Filipek.If you are interested in these two topics, stay tuned! References In the new decade, I am going to shift my focus to code safety and developer life. If you have been reading my articles over the years, you will notice they are usually performance-focused. You are welcome to build and run the benchmark with GCC and Clang. What about GCC and Clang? I do not have access to the latest C++ compilers on Linux. Trans Unord Map with string_view timing: 762ms Trans Unord Map with char* timing: 1506ms Normal Unord Map with char* timing: 2382ms Normal Unord Map with string timing: 738ms It should be a bug which I shall report to Microsoft. string_view has about the same performance as std::string that looks about right while const char* has worse performance in transparent map than normal map. Short text should be faster since std::string is implemented with Short String Optimization(SSO), meaning std::string has a short buffer that is used whenever the text is short enough to fit in that buffer, instead of allocating on the heap. The benchmark is built in Release 圆4 mode with /Ox, the highest compiler optimization. Std::cout < < " grandtotal:" < < grandtotal < < " <- Ignore this\n" < < std::endl įirst, we run the benchmark with short text. Stopwatch.start( " Trans Unord Map with string_view") Stopwatch.start( " Trans Unord Map with char*") Stopwatch.start( " Normal Unord Map with char*") Stopwatch.start( " Normal Unord Map with string") Stopwatch.start( " Trans Map with string_view") Stopwatch.start( " Trans Map with char*") Stopwatch.start( " Normal Map with char*") Stopwatch.start( " Normal Map with string") Initially, I was using the associative container's operator for search but it turns out that does not accept string_view, so find() is used.Ĭonst std::unordered_map & unordmapNormal,Ĭonst std::unordered_map & unordmapTrans) Their only purpose is to prevent the compiler from optimizing away the for loops since they are not doing any useful work. Ignore the total and grandtotal variables. UnordmapTrans.find( " Terry"sv) // no memory alloc UnordmapTrans.find( " Terry") // no memory alloc but strlen() is used Size_t operator()( std::string_view txt) const Using hash_type = std::hash // just a helper local type Using key_equal = std::equal_to // Pred to use Care must be taken with accessing string_view that the original std::string or character buffer, it pointed to, must be still in scope. What is a string_view? string_view is a non-owning view to a not-null-terminating character buffer with its length. To help you understand the sv suffix in the later code, it has to be noted whenever sv is appended to a string literal, we are telling the compiler to create a string_view from literal. Thanks, Billy O'Neal and VC++ team! In order to enable the latest C++ standard in VC++ to compile this benchmark, go to VC++ general property page and choose std:c++latest from the C++ Language Standard dropdown as shown below: However, Visual C++ team has already implemented some of C++20 library features. We are officially in 2020 but as of writing time, the C++20 Standard is yet to be ratified by C++ committee. C++20 introduced unordered transparency lookup that allows to do the same thing with unordered_map/unordered_set. Download cpp_hetero_lookup_bench-master_2.zip - 8.8 KBĬ++14 introduced ordered transparency lookup which enables const char* and string_view lookup without string instantiation on map/set objects.
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