如何将二进制文件转换为size_t？

如何解决如何将二进制文件转换为size_t？

我试图理解指针和二进制，但是我被困住了。我想将包含5个水果的列表写到二进制文件中。因此，我首先尝试编写（size_t）5，然后为每个水果块编写大小，然后是字符串本身。 这是我到目前为止所得到的：

// Write
vector<string> fruit_list = {"Apple","Pear","Banana","Cherry","Melon"};
ofstream fruits_out;
fruits_out.open("fruits.bin",std::ios::binary | std::ios::out);
size_t fruit_total = fruit_list.size();
fruits_out.write((char*)&fruit_total,sizeof(fruit_total));
for (const string& fruit : fruit_list){
    size_t fruit_size = fruit.size();
    fruits_out.write((char*)&fruit_size,sizeof(fruit_size));
    fruits_out.write(&fruit.c_str()[0],fruit_size);
}
fruits_out.close();

// Read
ifstream fruits_in;
fruits_in.open ("fruits.bin",std::ios::binary | std::ios::in);
char *buffer = new char;
fruits_in.read((char*)&buffer,sizeof(size_t));
cout << *buffer << endl;

我什至无法读取第一个值。 无论如何，请帮我，我已经坚持了很长时间。

解决方法

您的编写代码很好（减少了对错误的处理，并且当&c_str()[0]本身就足够时，无需使用c_str()）。

但是，您的阅读代码全错了。不仅要分配大小仅为1个字节的buffer，而且还要尝试向其中读取sizeof(size_t)个字节。但是更糟糕的是，您正在读buffer变量所在的堆栈空间，而不是读到buffer所指向的动态内存！

就像编写代码通过指向size_t的指针传递ostream::write()一样，您也可以通过指向size_t的指针传递istream::read()。

尝试更多类似的方法：

std::ostream& writeSizeT(std::ostream &out,size_t value)
{
    out.write(reinterpret_cast<char*>(&value),sizeof(value));

    /* alternatively:
    uint64_t temp = htobe64(value); // or equivalent...
    out.write(reinterpret_cast<char*>(&temp),sizeof(temp));
    */

    return out;
}

std::ostream& writeString(std::ostream &out,const std::string &value)
{
    size_t size = value.size();
    if ((writeSizeT(out,size)) && (size > 0)) {
        out.write(value.c_str(),size);
    }
    return out;
}

std::ostream& writeFruitList(std::ostream &out,const std::vector<std::string> &list)
{
    if (writeSizeT(out,list.size())) {
        for (const std::string &fruit : list){
            if (!writeString(out,fruit)) {
                break;
            }
        }
    }
    return out;
}

std::vector<std::string> fruit_list = {"Apple","Pear","Banana","Cherry","Melon"};
std::ofstream fruits_out("fruits.bin",std::ios::binary);
writeFruitList(fruits_out,fruit_list);
fruits_out.close();

std::istream& readSizeT(std::istream &in,size_t &value)
{
    in.read(reinterpret_cast<char*>(&value),sizeof(value));

    /* alternatively:
    uint64_t temp;
    if (in.read(reinterpret_cast<char*>(&temp),sizeof(temp))) {
        temp = be64toh(temp); // or equivalent...
        value = static_cast<size_t>(temp);
    }
    */

    return in;
}

std::istream& readString(std::istream &in,std::string &value)
{
    value.clear();
    size_t size;
    if ((readSizeT(in,size)) && (size > 0)) {
        value.resize(size);
        in.read(&value[0]/*or: value.data()*/,size);
    }
    return in;
}

std::istream& readFruitList(std::istream &in,std::vector<std::string> &list)
{
    list.clear();
    size_t size;
    if (readSizeT(in,size)) {
        list.reserve(size);
        std::string fruit;
        for (size_t i = 0; (i < size) && (readString(in,fruit)); ++i){
            list.push_back(std::move(fruit));
        }
    }
    return in;
}

std::vector<std::string> fruit_list;
std::ifstream fruits_in("fruits.bin",std::ios::binary);
readFruitList(fruits_in,fruit_list);
fruits_out.close();

for (const auto &fruit: fruit_list) {
    std::cout << fruit << std::endl;
}