为什么 {fmt} 比 std::stringstream 慢?

Question

从我在网上读到的内容来看，{fmt} 库应该比字符串流快得多。

但是，我做了一些简单的基准测试(测量系统时间，请参阅下面的代码(，似乎{fmt}总是比字符串流慢。我做错了什么吗？

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 O3 - 越少越差(是：

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

Answer 1

首先，我的机器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

二、为了得到一个公平的比较，替换

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

跟

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

这改善了(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