fstream读写函数错误

Question

我正在创建这个程序，它将一些项目存储到(.dat(文件中。。程序似乎运行得很顺利，但当我试图从文件中读取数据时，我在屏幕上看到了特殊字符，我试图保存的数据都没有正确保存，请注意，当我在main()中运行类似的代码时，它可以工作，它实际上向我显示了正确的输出。。请帮我一下，我真的不知道在这里该怎么办

void viewFile() {
Product item;
ifstream file("products.dat", ios::binary);
if (!file.read(reinterpret_cast<char*>(&item), sizeof(Product))) {
cout <<"failed to read";
system("exit");
}
item.viewProduct();
while (!file.eof()) {
item.viewProduct();
cout <<endl <<endl;
file.read((char *)(&item), sizeof(Product));
}
file.close();
}
void addProductToInventory() {
string name;
int quantity;
int pricePerItem;
ofstream obj("products.dat", ios::out | ios::binary);
Product item;
int numberOfProducts;
cout <<"enter the number of products you want to add in this inventory: ";
cin >> numberOfProducts;
for (int counter = 1; counter <= numberOfProducts; counter++) {
Product product;        
cout <<"enter the name of the object: ";
cin >> name;
cout <<"enter the quantity of the product: ";
cin >> quantity;
cout <<"enter the price of this product (per item): ";
cin >> pricePerItem;
product.setName(name);
product.setQuantity(quantity);
product.setPricePerItem(pricePerItem);
if (!obj.write(reinterpret_cast<char*>(&product), sizeof(Product))) {
cout <<"failed writingn";
system("exit");
}
cout <<"item addednn";
}
obj.flush();
obj.close();
}

这是我在main()中的代码，它的工作原理完全相同。。我想

ofstream file ("products.dat", ios::out | ios::binary | ios::trunc);
Product p1("hammer", 12, 3);
Product p2("screw driver", 43, 1);
if (!file.write(reinterpret_cast<char*>(&p1), sizeof(Product))) {
cout <<"failed to write";
system("exit");
}
file.write(reinterpret_cast<char*>(&p2), sizeof(Product));
file.close();
ifstream file2("products.dat", ios::out | ios::binary);
if (!file2.read(reinterpret_cast<char*>(&p1), sizeof(Product))) {
cout <<"failed to read";
system("exit");
}
while (!file2.eof()) {
p1.viewProduct();
cout <<endl <<endl;
file2.read((char *)(&p1), sizeof(Product));
}
file2.close();
}

p.S如果这是一个混乱的问题，我真的很抱歉。。我已经调试了好几个小时了，现在我甚至都想不清了。

Answer 1

让我们从思考指针如何工作开始。

char* a = new char[2];
a[0] = 'a';
a[1] = 'b';
char* b = a;
std::cout << a[0] << a[1]; //prints ab
std::cout << b[0] << b[1]; //prints ab
a[1] = 'c';
std::cout << b[0] << b[1]; //prints ac
delete[] a;
std::cout << b[0] << b[1]; //prints random garbage because the memory that b points at is deleted

让我们开始写这些指向文件的指针

char* a = new char[2];
a[0] = 'a';
a[1] = 'b';
ofstream outFile("products.dat", ios::binary | ios::trunc);
outFile.write(reinterpret_cast<char*>(&a), sizeof(a));
outFile.close();
{
ifstream inFile("products.dat", ios::binary);
char* b;
inFile.read(reinterpret_cast<char*>(&b), sizeof(b));
std::cout << b[0] << b[1]; //prints ab
a[1] = 'c';
std::cout << b[0] << b[1]; //prints ac
}
delete[] a;
{
ifstream inFile("products.dat", ios::binary);
char* b;
inFile.read(reinterpret_cast<char*>(&b), sizeof(b));
std::cout << b[0] << b[1];//prints random garbage because the memory that b points at is deleted
}

std:：string包含指向动态分配内存的指针。

你应该这样做。

void viewFile() {
ifstream file("products.dat", ios::binary);
while (!file.eof()) {
int nameLength = 0;
if (!file.read(reinterpret_cast<char*>(&nameLength), sizeof(nameLength))) {
cout <<"failed to read";
system("exit");
}
std::string name{ nameLength, '?' };
if (!file.read(name.data(), sizeof(char) * nameLength) {
cout <<"failed to read";
system("exit");
}
int quantity;
if (!file.read(reinterpret_cast<char*>(&quantity), sizeof(quantity)) {
cout <<"failed to read";
system("exit");
}
int pricePerItem;
if (!file.read(reinterpret_cast<char*>(&pricePerItem), sizeof(pricePerItem)) {
cout <<"failed to read";
system("exit");
}
Product item{ std::move(name),quantity, pricePerItem };
item.viewProduct();
}
file.close();
}
void addProductToInventory() {
string name;
int quantity;
int pricePerItem;
ofstream obj("products.dat", ios::out | ios::binary);
int numberOfProducts;
cout <<"enter the number of products you want to add in this inventory: ";
cin >> numberOfProducts;
for (int counter = 1; counter <= numberOfProducts; counter++) {   
cout <<"enter the name of the object: ";
cin >> name;
cout <<"enter the quantity of the product: ";
cin >> quantity;
cout <<"enter the price of this product (per item): ";
cin >> pricePerItem;
int nameLength = name.size();
if (!obj.write(reinterpret_cast<char*>(&nameLength), sizeof(nameLength))) {
cout <<"failed writingn";
system("exit");
}
if (!obj.write(name.data(), sizeof(char) * nameLength) {
cout <<"failed writingn";
system("exit");
}
if (!obj.write(reinterpret_cast<char*>(&quantity), sizeof(quantity))) {
cout <<"failed writingn";
system("exit");
}
if (!obj.write(reinterpret_cast<char*>(&pricePerItem), sizeof(pricePerItem))) {
cout <<"failed writingn";
system("exit");
}
cout <<"item addednn";
}
obj.flush();
obj.close();
}