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##### Comparing the performance of a single double_ generator

These performance measurements are centered around default formatting of a single `double` floating point number using different libraries and methods. The overall execution times for those examples are compared below. We compare using `sprintf`, C++ iostreams, Boost.Format, and Spirit.Karma.

For the full source code of the performance test please see here: double_performance.cpp. All the measurements have been done by executing `1e6` iterations for each formatting type (NUMITERATIONS is set to `1e6` in the code shown below).

Code used to measure the performance for `sprintf`:

    char buffer[256];
    for (int i = 0; i < NUMITERATIONS; ++i) {
        sprintf(buffer, "%f", 12345.12345);
    }

Code used to measure the performance for standard C++ iostreams:

    std::stringstream strm;
    for (int i = 0; i < NUMITERATIONS; ++i) {
        strm.str("");
        strm << 12345.12345;
    }

Code used to measure the performance for Boost.Format:

    std::string generated;
    boost::format double_format("%f");
    for (int i = 0; i < NUMITERATIONS; ++i)
        generated = boost::str(double_format % 12345.12345);

The following code shows the common definitions used by all Spirit.Karma performance measurements as listed below:

    using boost::spirit::karma::double_;

Code used to measure the performance for Spirit.Karma using a plain character buffer:

    char buffer[256];
    for (int i = 0; i < NUMITERATIONS; ++i) {
        char *p = buffer;
        generate(p, double_, 12345.12345);
        *p = '\0';
    }

The following table shows the overall performance results collected while using different compilers. All times are in seconds measured for `1e6` iterations (platform: Windows7, Intel Core Duo(tm) Processor, 2.8GHz, 4GByte RAM). For a more readable comparison of the results see this figure.

Table 6. Performance comparison for a single double (all times in [s], `1e6` iterations)

Library | gcc 4.4.0 (32 bit) | VC++ 10 (32 bit) | Intel 11.1 (32 bit) | gcc 4.4.0 (64 bit) | VC++ 10 (64 bit) | Intel 11.1 (64 bit)
--- | --- | --- | --- | --- | --- | ---
sprintf | 0.755 | 0.965 | 0.880 | 0.713 | 0.807 | 0.694
iostreams | 2.316 | 2.624 | 1.964 | 1.634 | 1.468 | 1.354
Boost.Format | 3.188 | 3.737 | 2.878 | 3.217 | 2.672 | 2.011
Spirit.Karma double_ | 0.813 | 0.561 | 0.368 | 0.426 | 0.260 | 0.218

Figure 4. Performance comparison for a single double