[英]C++ Reading fstream data from a file does not return correct values. inputFile.tellg returns -1
[英]C++, fstream, Reading from the file does not give correct result
我試圖創建一個簡單的程序,將輸入的數據寫入文件,然后從文件中讀取該數據。 由於某種原因,輸出不正確。 它從文件中讀取的第一個值顯示所有三個輸入值,而不僅僅是第一個。 有人可以告訴我我在做什么錯嗎?
#include<iostream>
#include<iomanip>
#include<string>
#include<fstream>
using namespace std;
int main() {
double totalRainfall=0.0, highTemperature,lowTemperature, totalHigh=0.0,
totalLow=0.0;
// open a file in write mode.
ofstream outFile;
outFile.open("statistics.txt");
cout << "Enter total rainfall (in inches) : ";
cin >> totalRainfall;
outFile <<totalRainfall; // write inputted data into the file.
cout << "Enter high temperature (in Fahrenheit): ";
cin >> highTemperature;
outFile << highTemperature; // write inputted data into the file.
cout << "Enter low temperature (in Fahrenheit) : ";
cin >> lowTemperature;
outFile << lowTemperature; // write inputted data into the file.
//close the opened file
outFile.close();
//open a file in read mode.
ifstream inFile;
inFile.open("statistics.txt");
inFile >> totalRainfall;
cout << "The rainfall: ";
cout<< totalRainfall<<endl;
inFile >> highTemperature;
cout << "High temperature: ";
cout << highTemperature<<endl;
inFile >> lowTemperature;
cout << "Low temperature: ";
cout<<lowTemperature<<endl;
// close the opened file.
inFile.close();
system("pause");
return 0;
}
您應該在每個輸出數字之間放置一個分隔符,以便在讀取文件時知道數字的開始位置和結束位置。 用作分隔符的空格字符允許讀取數字而無需特定代碼以跳過分隔符(讀取數字的STL函數將跳過前導空格字符)。
下面以空格作為分隔符的示例:
#include <fstream>
#include <iostream>
using namespace std;
const char sep = ' ';
const char* const file_path = "statistics.txt";
int main()
{
double value = 0.0;
ofstream outFile;
outFile.open(file_path);
cout << "Enter total rainfall (in inches): ";
cin >> value;
outFile << value << sep;
cout << "Enter high temperature (in Fahrenheit): ";
cin >> value;
outFile << value << sep;
cout << "Enter low temperature (in Fahrenheit): ";
cin >> value;
outFile << value;
outFile.close();
ifstream inFile;
inFile.open(file_path);
inFile >> value;
cout << "The rainfall: ";
cout << value << endl;
inFile >> value;
cout << "High temperature: ";
cout << value << endl;
inFile >> value;
cout << "Low temperature: ";
cout << value << endl;
return 0;
}
您應該使用fstream對象的write()
和read()
方法。 這些方法允許您寫入/讀取特定數量的字節。 因此,您可以只創建兩個實用程序函數,如下所示:
void writeValue(ofstream& f, const double& v) {
f.write(reinterpret_cast<const char*>(&v), sizeof(v));
}
void readValue(ifstream& f, double& v) {
f.read(reinterpret_cast<char*>(&v), sizeof(v));
}
現在,您可以在主要代碼中使用它們:
double value = 0.0;
ofstream outFile;
outFile.open(file_path);
cout << "Enter total rainfall (in inches): ";
cin >> value;
writeValue(outFile, value);
cout << "Enter high temperature (in Fahrenheit): ";
cin >> value;
writeValue(outFile, value);
cout << "Enter low temperature (in Fahrenheit): ";
cin >> value;
writeValue(outFile, value);
outFile.close();
ifstream inFile;
inFile.open(file_path);
readValue(inFile, value);
cout << "The rainfall: ";
cout << value << endl;
readValue(inFile, value);
cout << "High temperature: ";
cout << value << endl;
readValue(inFile, value);
cout << "Low temperature: ";
cout << value << endl;
我上面提到的實用程序功能只能處理double
值。 這不靈活,如果您想擴展代碼並處理不同類型的信息,就應該成為一個問題。 因此,如果您想使代碼更健壯並在序列化文件中允許其他變量類型(例如,int,float),則可以使用模板代替:
template<typename T>
void writeValue(ofstream& f, const T& v) {
f.write(reinterpret_cast<const char*>(&v), sizeof(v));
}
template<typename T>
constexpr void readValue(ifstream& f, T& v) {
f.read(reinterpret_cast<char*>(&v), sizeof(v));
}
如果您還想處理std::string
,則可以為該特定類型自定義模板:
template<>
constexpr void writeValue(ofstream& f, const std::string& v) {
auto len = v.length();
f.write(reinterpret_cast<const char*>(&len), sizeof(len));
f.write(v.c_str(), v.length() * sizeof(char));
}
template<>
void readValue(ifstream& f, std::string& v) {
std::string::size_type len;
f.read(reinterpret_cast<char *>(&len), sizeof(len));
char buffer[len + 1];
f.read(buffer, len);
buffer[len] = '\0';
v = buffer;
}
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