将文件内容读入 C++ 中的字符串
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Read file-contents into a string in C++
提问by sonofdelphi
Possible Duplicate:
What is the best way to slurp a file into a std::string in c++?
In scripting languages like Perl, it is possible to read a file into a variable in one shot.
在 Perl 等脚本语言中，可以一次性将文件读入变量。
open(FILEHANDLE,$file);
$content=<FILEHANDLE>;
What would be the most efficient way to do this in C++?
在 C++ 中执行此操作的最有效方法是什么？
回答by Maik Beckmann
Like this:
像这样：
#include <fstream>
#include <string>
int main(int argc, char** argv)
{
std::ifstream ifs("myfile.txt");
std::string content( (std::istreambuf_iterator<char>(ifs) ),
(std::istreambuf_iterator<char>() ) );
return 0;
}
The statement
该声明
std::string content( (std::istreambuf_iterator<char>(ifs) ),
(std::istreambuf_iterator<char>() ) );
can be split into
可以分为
std::string content;
content.assign( (std::istreambuf_iterator<char>(ifs) ),
(std::istreambuf_iterator<char>() ) );
which is useful if you want to just overwrite the value of an existing std::string variable.
如果您只想覆盖现有 std::string 变量的值，这很有用。
回答by M. Williams
The most efficient, but not the C++ way would be:
最有效但不是 C++ 的方式是：
FILE* f = fopen(filename, "r");
// Determine file size
fseek(f, 0, SEEK_END);
size_t size = ftell(f);
char* where = new char[size];
rewind(f);
fread(where, sizeof(char), size, f);
delete[] where;
#EDIT - 2
#编辑 - 2
Just tested the std::filebufvariant also. Looks like it can be called the best C++ approach, even though it's not quite a C++ approach, but more a wrapper. Anyway, here is the chunk of code that works almost as fast as plain C does.
也刚刚测试了该std::filebuf变体。看起来它可以被称为最好的 C++ 方法，尽管它不是一种 C++ 方法，而更像是一个包装器。无论如何，这里的代码块几乎和普通 C 一样快。
std::ifstream file(filename, std::ios::binary);
std::streambuf* raw_buffer = file.rdbuf();
char* block = new char[size];
raw_buffer->sgetn(block, size);
delete[] block;
I've done a quick benchmark here and the results are following. Test was done on reading a 65536Kbinary file with appropriate (std::ios:binaryand rb) modes.
我在这里做了一个快速基准测试，结果如下。测试是在读取具有适当 (和) 模式的65536K二进制文件时完成的。std::ios:binaryrb
[==========] Running 3 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 4 tests from IO
[ RUN ] IO.C_Kotti
[ OK ] IO.C_Kotti (78 ms)
[ RUN ] IO.CPP_Nikko
[ OK ] IO.CPP_Nikko (106 ms)
[ RUN ] IO.CPP_Beckmann
[ OK ] IO.CPP_Beckmann (1891 ms)
[ RUN ] IO.CPP_Neil
[ OK ] IO.CPP_Neil (234 ms)
[----------] 4 tests from IO (2309 ms total)
[----------] Global test environment tear-down
[==========] 4 tests from 1 test case ran. (2309 ms total)
[ PASSED ] 4 tests.
回答by Martin York
The most efficient is to create a buffer of the correct size and then read the file into the buffer.
最有效的方法是创建一个正确大小的缓冲区，然后将文件读入缓冲区。
#include <fstream>
#include <vector>
int main()
{
std::ifstream file("Plop");
if (file)
{
/*
* Get the size of the file
*/
file.seekg(0,std::ios::end);
std::streampos length = file.tellg();
file.seekg(0,std::ios::beg);
/*
* Use a vector as the buffer.
* It is exception safe and will be tidied up correctly.
* This constructor creates a buffer of the correct length.
* Because char is a POD data type it is not initialized.
*
* Then read the whole file into the buffer.
*/
std::vector<char> buffer(length);
file.read(&buffer[0],length);
}
}
回答by Draco Ater
There should be no \0in text files.
\0文本文件中应该没有。
#include<iostream>
#include<fstream>
using namespace std;
int main(){
fstream f(FILENAME, fstream::in );
string s;
getline( f, s, 'unsigned int FileRead( std::istream & is, std::vector <char> & buff ) {
is.read( &buff[0], buff.size() );
return is.gcount();
}
void FileRead( std::ifstream & ifs, string & s ) {
const unsigned int BUFSIZE = 64 * 1024; // reasoable sized buffer
std::vector <char> buffer( BUFSIZE );
while( unsigned int n = FileRead( ifs, buffer ) ) {
s.append( &buffer[0], n );
}
}
');
cout << s << endl;
f.close();
}
回答by Draco Ater
This depends on a lot of things, such as what is the size of the file, what is its type (text/binary) etc. Some time ago I benchmarked the following function against versions using streambuf iterators - it was about twice as fast:
这取决于很多事情，例如文件的大小是什么，它的类型是什么（文本/二进制）等。前段时间我使用流缓冲迭代器对以下函数的版本进行了基准测试 - 它的速度大约是其两倍：
#include<iostream>
#include<vector>
#include<iterator>
main(int argc,char *argv[]){
// read standard input into vector:
std::vector<char>v(std::istream_iterator<char>(std::cin),
std::istream_iterator<char>());
std::cout << "read " << v.size() << "chars\n";
}
回答by catwalk
maybe not the most efficient, but reads data in one line:
也许不是最有效的，但在一行中读取数据：
ifstream file("file", ios::binary);
string fileStr;
istreambuf_iterator<char> inputIt(file), emptyInputIt
back_insert_iterator<string> stringInsert(fileStr);
copy(inputIt, emptyInputIt, stringInsert);
回答by academicRobot
Here's an iterator-based method.
这是一个基于迭代器的方法。
##代码##