为什么{fmt}比std :: stringstream慢？

Question

根据我在网上阅读的内容，fmt库的运行速度要比例如字符串流。

但是，我做了一些简单的基准测试（测量系统时间，请参见下面的代码），看来fmt总是比例如字符串流。我有什么问题吗？

uint64_t start;
uint64_t stop;

long MAXCOUNT = 10000000;

std::srand(123);

int* numbers =  new int[MAXCOUNT];
for ( int i = 0; i < MAXCOUNT; i++) {
    numbers[i] = std::rand();
}

{
    std::string result;
    start = currentTimeInMillis();
    for ( int i = 0; i < MAXCOUNT; i++) {
        result += fmt::format("Number {} is great!", numbers[i]);
    }
    stop = currentTimeInMillis();
    fmt::print("timing fmt : {} ms   /   string length: {}\n", stop-start, result.size());
}


{
    std::string result;
    std::stringstream ss;
    start = currentTimeInMillis();
    for ( int i = 0; i < MAXCOUNT; i++) {
        ss << "Number " << numbers[i] << " is great!";
    }
    result = ss.str();
    stop = currentTimeInMillis();
    fmt::print("timing stds: {} ms   /   string length: {}\n", stop-start, result.size());
}

此代码的典型结果（优化级别为O3-更少，甚至更糟）是：

timing fmt : 1414 ms   /   string length: 264823200
timing stds: 1287 ms   /   string length: 264823200

Answer 1

首先，我在计算机上没有得到相同的数字，对我来说fmt更快：

timing fmt : 1713 ms   /   string length: 264825935
timing stds: 2483 ms   /   string length: 264825935

第二，为了公平起见，请替换

result += fmt::format("Number {} is great!", numbers[i]);

with

fmt::format_to(std::back_inserter(result), "Number {} is great!", numbers[i]);

这可以缩短（再次在我的机器上）的时间

timing fmt : 1153 ms   /   string length: 264825935
timing stds: 2482 ms   /   string length: 264825935