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Most efficient way to assign values to maps

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北荒
北荒 2021-02-07 19:30

Which way to assign values to a map is most efficient? Or are they all optimized to the same code (on most modern compilers)?

   // 1) Assignment using array ind
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  • 野趣味
    野趣味 (楼主)
    2021-02-07 20:03

    Even though there has been a couple of good answers already I thought I might as well do a quick benchmark. Ran each one 5 million times and used c++11's chrono to measure the time it took.

    Heres the code:

    #include 
#include 
#include 
#include 

// 5 million
#define times 5000000

int main()
{
    std::map foo1, foo2, foo3, foo4, foo5;
    std::chrono::steady_clock::time_point timeStart, timeEnd;
    int x = 0;

    // 1) Assignment using array index notation
    timeStart = std::chrono::steady_clock::now();
    for (x = 0; x <= times; x++)
    {
        foo1[std::to_string(x)] = 12345;
    }
    timeEnd = std::chrono::steady_clock::now();
    printf("1) took %i milliseconds\n", (unsigned long long)std::chrono::duration_cast(timeEnd-timeStart).count());

    // 2) Assignment using member function insert() and STL pair
    timeStart = std::chrono::steady_clock::now();
    for (x = 0; x <= times; x++)
    {
        foo2.insert(std::pair(std::to_string(x), 12345));
    }
    timeEnd = std::chrono::steady_clock::now();
    printf("2) took %i milliseconds\n", (unsigned long long)std::chrono::duration_cast(timeEnd-timeStart).count());

    // 3) Assignment using member function insert() and "value_type()"
    timeStart = std::chrono::steady_clock::now();
    for (x = 0; x <= times; x++)
    {
        foo3.insert(std::map::value_type(std::to_string(x), 12345));
    }
    timeEnd = std::chrono::steady_clock::now();
    printf("3) took %i milliseconds\n", (unsigned long long)std::chrono::duration_cast(timeEnd-timeStart).count());

    // 4) Assignment using member function insert() and "make_pair()"
    timeStart = std::chrono::steady_clock::now();
    for (x = 0; x <= times; x++)
    {
        foo4.insert(std::make_pair(std::to_string(x), 12345));
    }
    timeEnd = std::chrono::steady_clock::now();
    printf("4) took %i milliseconds\n", (unsigned long long)std::chrono::duration_cast(timeEnd-timeStart).count());

    // 5) Matthieu M.'s suggestion of C++11's emplace
    timeStart = std::chrono::steady_clock::now();
    for (x = 0; x <= times; x++)
    {
        foo5.emplace(std::to_string(x), 12345);
    }
    timeEnd = std::chrono::steady_clock::now();
    printf("5) took %i milliseconds\n", (unsigned long long)std::chrono::duration_cast(timeEnd-timeStart).count());

    return 0;
}

    The output for 5 million iterations is:

    1) took 23448 milliseconds
2) took 22854 milliseconds
3) took 22372 milliseconds
4) took 22988 milliseconds
5) took 21356 milliseconds

    GCC version:

    g++ (Built by MinGW-builds project) 4.8.0 20121225 (experimental)

    My machine:

    Intel i5-3570k overclocked at 4.6 GHz

    EDIT1: Fixed the code and redid the benchmark.

    EDIT2: Added Matthieu M.'s suggestion for C++11's emplace and he is right, emplace is fastest

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