在信息学竞赛中，输入数据规模可能会很大，这时候就需要注意文件读取的效率。本文在 Linux 环境下测试了 C++ 几种常见读入方式的效率。

1. 系统环境

Arch Linux x86_64

默认 Linux 内核，版本 6.1.3

gcc 12.2.0

ext4

2. 测试代码

编译命令（省略文件）：g++ -std=c++20，不开优化。

2.0 随机整数（32位有符号）生成

#include <climits>
#include <fstream>
#include <random>
using namespace std;
using i64 = long long;
int main()
{
    constexpr int N = 10000;
    ofstream fout("in");
    random_device rd;
    mt19937 gen(rd());
    for (int i = 0; i < N; ++i) {
        for (int j = 0; j < N; ++j)
            fout << (i64)gen() + (i64)INT_MIN << ' ';
        fout << '\n';
    }
    return 0;
}

2.1 freopen + scanf

#include <cstdio>
using namespace std;
int main()
{
    freopen("in", "r", stdin);
    constexpr int N = 10000;
    int x;
    for (int i = 0; i < N; ++i)
        for (int j = 0; j < N; ++j)
            scanf("%d", &x);
    return 0;
}

2.2 FILE*

#include <cstdio>
using namespace std;
int main()
{
    FILE *fp = fopen("in", "r");
    constexpr int N = 10000;
    int x;
    for (int i = 0; i < N; ++i)
        for (int j = 0; j < N; ++j)
            fscanf(fp, "%d", &x);
    return 0;
}

2.3 ifstream

#include <fstream>
using namespace std;
int main()
{
    ifstream fin("in");
    constexpr int N = 10000;
    int x;
    for (int i = 0; i < N; ++i)
        for (int j = 0; j < N; ++j)
            fin >> x;
    return 0;
}

2.4 freopen + cin

#include <cstdio>
#include <iostream>
using namespace std;
int main()
{
    freopen("in", "r", stdin);
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    constexpr int N = 10000;
    int x;
    for (int i = 0; i < N; ++i)
        for (int j = 0; j < N; ++j)
            cin >> x;
    return 0;
}

2.5 快读

从洛谷模板题快速排序的最优解复制的快读模板（做了少量修改）

#include <cctype>
#include <cstdio>
#include <cstring>
using namespace std;
namespace IO
{
class qistream
{
    static const size_t SIZE = 1 << 16, BLOCK = 32;
    FILE *fp;
    char buf[SIZE];
    int p;
  public:
    qistream(FILE *_fp = stdin) : fp(_fp), p(0)
    {
        fread(buf + p, 1, SIZE - p, fp);
    }
    void flush()
    {
        memmove(buf, buf + p, SIZE - p), fread(buf + SIZE - p, 1, p, fp), p = 0;
    }
    qistream &operator>>(char &str)
    {
        str = getch();
        while (isspace(str))
            str = getch();
        return *this;
    }
    template <class T> qistream &operator>>(T &x)
    {
        x = 0;
        p + BLOCK >= SIZE ? flush() : void();
        bool flag = false;
        for (; !isdigit(buf[p]); ++p)
            flag = buf[p] == '-';
        for (; isdigit(buf[p]); ++p)
            x = x * 10 + buf[p] - '0';
        x = flag ? -x : x;
        return *this;
    }
    char getch() { return buf[p++]; }
    qistream &operator>>(char *str)
    {
        char ch = getch();
        while (ch <= ' ')
            ch = getch();
        for (int i = 0; ch > ' '; ++i, ch = getch())
            str[i] = ch;
        return *this;
    }
};
} // namespace IO
int main()
{
    FILE *fp = fopen("in", "r");
    IO::qistream fin(fp);
    constexpr int N = 10000;
    int x;
    for (int i = 0; i < N; ++i)
        for (int j = 0; j < N; ++j)
            fin >> x;
    return 0;
}