How to determine if a file exists using C++?

Methods to determine whether a file exists in C: Use the ifstream class to successfully open the file to indicate existence; use the fopen() function to return a non-null pointer to indicate existence; use the std::filesystem::exists() function to directly check the file does it exist.

Determining whether a file exists using C

Determining whether a file exists is a common task in programming. There are several ways to do this in C.

Method 1: Use ifstream

ifstream class is used to read files. If the file exists, the ifstream object will open successfully; otherwise, the open will fail.

#include <fstream>

int main() {
  std::ifstream file("myfile.txt");
  if (file.is_open()) {
    // 文件存在
  } else {
    // 文件不存在
  }
  file.close();
  return 0;
}

Method 2: Use the fopen()

fopen() function to open the file in read-only mode. If the file exists, fopen() will return a pointer to the file; otherwise, NULL will be returned.

#include <stdio.h>

int main() {
  FILE *file = fopen("myfile.txt", "r");
  if (file != NULL) {
    // 文件存在
    fclose(file);
  } else {
    // 文件不存在
  }
  return 0;
}

Method 3: Use std::filesystem::exists()

C 17 introduced the std::filesystem library, which contains exists() Function to check whether the file exists.

#include <iostream>
#include <filesystem>

int main() {
  std::filesystem::path path("myfile.txt");
  if (std::filesystem::exists(path)) {
    // 文件存在
  } else {
    // 文件不存在
  }
  return 0;
}

Practical case

Suppose you have a file named myfile.txt. To check whether it exists, you can use the following code snippet :

#include <ifstream>

int main() {
  std::ifstream file("myfile.txt");
  if (file.is_open()) {
    // 在此处处理文件存在的逻辑
    std::cout << "文件存在" << std::endl;
  } else {
    // 在此处处理文件不存在的逻辑
    std::cout << "文件不存在" << std::endl;
  }
  file.close();
  return 0;
}

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