C++使用ifstream读取文件内容的示例详解

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

//读取整个文件内容到char array数组中去
void fileReadAllToCharArray()
{
	std::ifstream file;
	//以只读方式打开文件
	file.open("myfile.txt", std::ios::in);

	//指针定位到文件末尾
	file.seekg(0, std::ios::end);
	int fileLength = file.tellg();

	//指定定位到文件开始
	file.seekg(0, std::ios::beg);

	cout << "fileLength:" << fileLength << endl;
	char* buffer = new char[fileLength + 1];
	file.read(buffer, fileLength);
	buffer[fileLength] = '\0';
	string contents = buffer;
	cout << "contents:" << contents << endl;

	if (buffer) {
		delete[] buffer;
	}
	file.close();
}

//读取方式:逐行读取Line by Line, 将行读入字符数组, 行之间用回车换行区分
void fileReadToCharArray()
{
	std::ifstream file("myfile.txt");
	
	constexpr int LINE_LENGTH = 100;
	char str[LINE_LENGTH];

	int lineNum = 0;
	while (file.getline(str, LINE_LENGTH))
	{
		cout << "Read from line[" << ++lineNum << "] :"<<str<<endl;
	}
	cout << "file has line:" << lineNum << endl;
}

//读取方式:逐行读取Line by Line, 将行读入string, 行之间用回车换行区分
void fileReadToString()
{
	std::ifstream file("myfile.txt");

	int lineNum = 0;
	string str;
	while (getline(file, str)) {
		cout << "Read Data on Line:[" << ++lineNum<<"] :" << str <<endl;
	}
	cout << "file has line:" << lineNum << endl;
}

//读取方式：逐词读取Word by Word，词之间用空格划分
void fileReadWbW()
{
	std::ifstream file("myfile.txt");
	string s;
	while (file >> s)
	{
		cout << "Read From File[" << s <<"]"<<endl;
	}
}

//带检测文件名功能
void fileReadWithErrCheck()
{
	string fileName = "file .dat";
	std::ifstream fin(fileName.c_str());
	if (!fin) {
		cout << "Error Opening file:[" << fileName << "]" << " for input " << endl;
		exit(-1);
	}
}
int main()
{
#if  0
	char data[100];

	ofstream outfile;
	outfile.open("myfile.txt", ios::out | ios::trunc);

	cout << "enter your name: ";
	//cin.getline(data, 100);
	outfile << "hello world"<<endl;

	ifstream infile;
	infile.open("myfile.txt", ios::in);
	cout << "read file from myfile.txta" << endl;
	string readData;
	infile >> readData;
	std::cout << "data:" << readData << endl;
	outfile.close();
#endif

	//读取整个文件内容到char array数组中去
	fileReadAllToCharArray();
	std::cout << "-----------------" << endl;

	//逐行读取Line by Line
	fileReadToCharArray();
	std::cout << "-----------------" << endl;

	//文件逐词读取Word by Word
	fileReadWbW();
	std::cout << "-----------------" << endl;

	//逐行读取Line by Line, 将行读入string分
	fileReadToString();

	//带检测文件名功能
	fileReadWithErrCheck();
	std::cout << "-----------------" << endl;
}