在 C++ 中，文件操作主要通过 <fstream> 头文件提供的流类实现。以下是主要类和打开方式：
一、核心流类
ifstream 用于读取文件（输入流），继承自 istream $$ \\text{ifstream} \\subset \\text{istream} $$
ofstream 用于写入文件（输出流），继承自 ostream $$ \\text{ofstream} \\subset \\text{ostream} $$
fstream 支持读写操作（双向流），继承自 iostream $$ \\text{fstream} \\subset \\text{iostream} $$
二、文件打开模式
通过位或运算符 | 组合模式标志：
std::ios::in // 读模式（默认 ifstream）
std::ios::out // 写模式（默认 ofstream）
std::ios::app // 追加写入（尾部插入）
std::ios::ate // 打开后定位到文件尾
std::ios::trunc // 清空文件（默认 ofstream）
std::ios::binary // 二进制模式
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三、打开方式示例
1. 写入文件（覆盖模式）
#include <fstream>
int main() {
std::ofstream file("data.txt", std::ios::out | std::ios::trunc);
if (file.is_open()) {
file << "Hello, C++!" << std::endl;
file.close();
}
return 0;
}
2. 追加写入
std::ofstream file("log.txt", std::ios::app);
file << "New log entry" << std::endl;
3. 读取文件
std::ifstream file("config.ini");
std::string line;
while (std::getline(file, line)) {
std::cout << line << std::endl;
}
4. 读写混合操作
std::fstream file("data.db",
std::ios::in | std::ios::out |
std::ios::binary);
四、错误检测
if (!file) {
std::cerr << "文件打开失败" << std::endl;
}
// 检查流状态
if (file.fail()) { /* 操作失败处理 */ }
if (file.eof()) { /* 到达文件尾 */ }
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五、二进制文件操作
struct Record { int id; char name[32]; };
// 写入二进制数据
Record r = {1, "Alice"};
file.write(reinterpret_cast<char*>(&r), sizeof(Record));
// 读取二进制数据
Record data;
file.read(reinterpret_cast<char*>(&data), sizeof(Record));
注意：二进制操作需确保数据内存布局对齐，跨平台时需谨慎处理字节序。
