从输入迭代器创建 C++ std::string 的性能

2022-01-07 00:00:00 string stream c++

我正在做一些非常简单的事情:将整个文本文件从磁盘拖入 std::string.我当前的代码基本上是这样做的:

I'm doing something really simple: slurping an entire text file from disk into a std::string. My current code basically does this:

std::ifstream f(filename);
return std::string(std::istreambuf_iterator<char>(f), std::istreambuf_iterator<char>());

这不太可能对程序产生任何性能影响，但我仍然很好奇这是否是一种缓慢的方式.

It's very unlikely that this will ever have any kind of performance impact on the program, but I still got curious whether this is a slow way of doing it.

字符串的构造是否有涉及大量重新分配的风险?使用 seekg()/tellg() 计算文件大小和 reserve() 会不会更好(也就是更快)> 在读取之前字符串中有那么多空间?

Is there a risk that the construction of the string will involve a lot of reallocations? Would it be better (that is, faster) to use seekg()/tellg() to calculate the size of the file and reserve() that much space in the string before doing the reading?

推荐答案

我对您的实现 (1)、我的 (2) 以及我在 stackoverflow 上找到的其他两个(3 和 4)进行了基准测试.

I benchmarked your implementation(1), mine(2), and two others(3 and 4) that I found on stackoverflow.

结果(100 次运行的平均值；使用 gettimeofday 计时，文件是 40 段 lorem ipsum):

Results (Average of 100 runs; timed using gettimeofday, file was 40 paragraphs of lorem ipsum):

  • readFile1:764
  • readFile2:104
  • readFile3:129
  • readFile4:402

实现:

string readFile1(const string &fileName)
{
    ifstream f(fileName.c_str());
    return string(std::istreambuf_iterator<char>(f),
            std::istreambuf_iterator<char>());
}

string readFile2(const string &fileName)
{
    ifstream ifs(fileName.c_str(), ios::in | ios::binary | ios::ate);

    ifstream::pos_type fileSize = ifs.tellg();
    ifs.seekg(0, ios::beg);

    vector<char> bytes(fileSize);
    ifs.read(&bytes[0], fileSize);

    return string(&bytes[0], fileSize);
}

string readFile3(const string &fileName)
{
    string data;
    ifstream in(fileName.c_str());
    getline(in, data, string::traits_type::to_char_type(
                      string::traits_type::eof()));
    return data;
}

string readFile4(const std::string& filename)
{
    ifstream file(filename.c_str(), ios::in | ios::binary | ios::ate);

    string data;
    data.reserve(file.tellg());
    file.seekg(0, ios::beg);
    data.append(istreambuf_iterator<char>(file.rdbuf()),
                istreambuf_iterator<char>());
    return data;
}

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