首先使用 mmap 映射文件到内存中，然后返回该文件对应的 memory 首地址。 char* data。

然后对该 char* 类型构造一个 ifstream（不复制 data 内容到 ifstream 中）。

代码如下：

#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <unistd.h>
#include <streambuf>
#include <istream>
#include <numeric>

struct membuf: std::streambuf {
    membuf(char const* base, size_t size) {
        char* p(const_cast<char*>(base));
        this->setg(p, p, p + size);
    }
};
struct imemstream: virtual membuf, std::istream {
    imemstream(char const* base, size_t size)
        : membuf(base, size)
        , std::istream(static_cast<std::streambuf*>(this)) {
    }
};

int main () {

    int fd = open("../XXXX", O_RDONLY, (mode_t)0600);
    if (fd == -1)
    {
        perror("Error opening file for writing");
        exit(EXIT_FAILURE);
    }        
    
    struct stat fileInfo = {0};
    if (fstat(fd, &fileInfo) == -1)
    {
        perror("Error getting the file size");
        exit(EXIT_FAILURE);
    }
    
    if (fileInfo.st_size == 0)
    {
        fprintf(stderr, "Error: File is empty, nothing to do\n");
        exit(EXIT_FAILURE);
    }
    printf("File size is %ji\n", (intmax_t)fileInfo.st_size);
    
    char *map = (char*)mmap(0, fileInfo.st_size, PROT_READ, MAP_SHARED, fd, 0);
    if (map == MAP_FAILED)
    {
        close(fd);
        perror("Error mmapping the file");
        exit(EXIT_FAILURE);
    }
    
    unsigned int inserted = 0;

    // 读取数据的开始
    string line;
    imemstream in(map, fileInfo.st_size);
    auto started = std::chrono::high_resolution_clock::now();
    // 这里遇到空格和换行都会当做一个 string 的结束
    while(in >> line && inserted < 100000) {
        std::cout << line << std::endl;
    }
    
    auto done = std::chrono::high_resolution_clock::now();
    double time_in_ms = std::chrono::duration_cast<std::chrono::milliseconds>(done-started).count();
    std::cout << "Total " << time_in_ms <<" ms" << std::endl;
    
    // Don't forget to free the mmapped memory
    if (munmap(map, fileInfo.st_size) == -1)
    {
        close(fd);
        perror("Error un-mmapping the file");
        exit(EXIT_FAILURE);
    }

    // Un-mmaping doesn't close the file, so we still need to do that.
    close(fd);
    
    return 0;
}

怎样快速读取文件

参考