[英]Convert bytes from a file to an integer c++
我正在嘗試解析一個.dat
文件,用此代碼逐字節讀取它。(文件名在arv [1]中)
std::ifstream is (arv[1], std::ifstream::binary);
if (is) {
is.seekg (0, is.end);
int length = is.tellg();
is.seekg (0, is.beg);
char * buffer = new char [length];
is.read (buffer,length);
if (is)
std::cout << "all characters read successfully.";
else
std::cout << "error: only " << is.gcount() << " could be read";
is.close();
}
現在所有文件都在buffer
變量中。 該文件包含以32位表示的數字,如何遍歷一次讀取4個字節的緩沖區並將其轉換為整數?
首先,存在內存泄漏,您動態分配了字符數組,但從未刪除[]它們。 使用std::string
代替:
std::string buffer(length,0);
is.read (&buffer[0],length);
現在,假設您已正確寫入整數並將其正確讀取到緩沖區中,則可以將此字符數組用作指向整數的指針:
int myInt = *(int*)&buffer[0];
(您知道為什么嗎?)如果您存儲了一個以上的整數:
std::vector<int> integers;
for (int i=0;i<buffer.size();i+=sizeof(int)){
integers.push_back(*(int*)&buffer[i]);
}
代替:
char * buffer = new char [length];
is.read (buffer,length);
您可以使用:
int numIntegers = length/sizeof(int);
int* buffer = new int[numIntegers];
is.read(reinterpret_cast<char*>(buffer), numIntegers*sizeof(int));
更新,以回應OP的評論
我建議的方法沒有任何問題。 這是一個示例程序,我使用g ++ 4.9.2看到的輸出。
#include <iostream>
#include <fstream>
#include <cstdlib>
void writeData(char const* filename, int n)
{
std::ofstream out(filename, std::ios::binary);
for ( int i = 0; i < n; ++i )
{
int num = std::rand();
out.write(reinterpret_cast<char*>(&num), sizeof(int));
}
}
void readData(char const* filename)
{
std::ifstream is(filename, std::ifstream::binary);
if (is)
{
is.seekg (0, is.end);
int length = is.tellg();
is.seekg (0, is.beg);
int numIntegers = length/sizeof(int);
int* buffer = new int [numIntegers];
std::cout << "Number of integers: " << numIntegers << std::endl;
is.read(reinterpret_cast<char*>(buffer), numIntegers*sizeof(int));
if (is)
std::cout << "all characters read successfully." << std::endl;
else
std::cout << "error: only " << is.gcount() << " could be read" << std::endl;
for (int i = 0; i < numIntegers; ++i )
{
std::cout << buffer[i] << std::endl;
}
}
}
int main()
{
writeData("test.bin", 10);
readData("test.bin");
}
產量
Number of integers: 10
all characters read successfully.
1481765933
1085377743
1270216262
1191391529
812669700
553475508
445349752
1344887256
730417256
1812158119
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