我用C++写了一个小文件处理类
它有一个管理RAII文件关闭的基类和两个派生类，一个用于写，一个用于读。衍生类有2个方法，分别是写或读值和写或读向量。
#include <vector>
#include <fstream>
#include <iostream>
#include <algorithm>
class File{
protected:
std::streampos offset;
std::fstream file;
public:
File(){
offset = 0;
}
virtual ~File(){
file.close();
}
};
class oFile : public File{
public:
oFile(std::string const& filename) {
file.open(filename, std::ios::out|std::ios::binary);
}
template <typename T>
void write_value(T value){
file.seekp(offset);
file.write(reinterpret_cast<char*> (&value), sizeof(T));
offset += sizeof(T);
std::cout<< "wrote a value: " << value << " current offset: " << offset << std::endl;
}
template <typename T>
void write_vector(std::vector<T> v){
write_value(v.size());
std::for_each(v.begin(), v.end(), [this](T& i){write_value(i);});
}
};
class iFile : public File{
public:
iFile(std::string const& filename){
file.open(filename, std::ios::in|std::ios::binary);
}
template <typename T>
void read_value(T& v){
file.seekg(offset);
file.read(reinterpret_cast<char*> (&v), sizeof(T));
offset += sizeof(T);
std::cout << "value read: " << v << " current offset: " << offset << std::endl;
}
template <typename T>
void read_vector(std::vector<T>& v){
typename std::vector<T>::size_type vsize;
read_value(vsize);
std::cout << "size of vector: " << vsize << std::endl;
for(typename std::vector<T>::size_type i = 0; i < vsize; ++i){
T value;
read_value(value);
v.push_back(value);
}
}
};
为了测试这些类，我写了这个函数。
oFile file("test.ndf");
double b = 12.;
file.write_value(b);
std::vector<double> v{1., 2., 3., 4., 6., 10., 15.};
file.write_vector(v);
iFile ifile("test.ndf");
double ib;
ifile.read_value(ib);
std::cout << ib << std::endl;
std::vector<double> iv;
ifile.read_vector(iv);
std::cout << "vector size: " << iv.size() << std::endl;
for(auto val : iv){
std::cout << val << " ";
}
std::cout << "last val: " << iv[iv.size() - 1] << std::endl;
}
但输出有问题
wrote a value: 7 offset: 12
wrote a value: 1 offset: 20
wrote a value: 2 offset: 28
wrote a value: 3 offset: 36
wrote a value: 4 offset: 44
wrote a value: 6 offset: 52
wrote a value: 10 offset: 60
wrote a value: 15 offset: 68 <--- tells me he wrote 15
value read: 12 offset: 8
12
value read: 7 offset: 12
size of vector: 7
value read: 1 offset: 20
value read: 2 offset: 28
value read: 3 offset: 36
value read: 4 offset: 44
value read: 6 offset: 52
value read: 10 offset: 60
value read: 10 offset: 68 <--- problem
vector size: 7
1 2 3 4 6 10 10 last val: 10 <--- last value should be 15
Hello world!
输出的最后一行是向量内容，虽然在写法中告诉我写了正确的值（15），但最后的值是重复的。
我不明白为什么它要写或读最后一个值2次？我的文件处理程序解决方案好吗？有更好的方法吗？
使用 MinGW (v9.2.0) 用 Code::Blocks (v20.03) 编译。
我的文件处理方案好吗？
RAII 资源管理是一个很好的、值得推荐的方法。
然而，对于RAII来说，变量的范围（以及生命周期）应该被更加仔细地考虑。
在OP.NET的暴露代码中，变量的范围（以及因此而产生的寿命）应该更加仔细地考虑。
int main()
{
oFile file("test.ndf");
double b = 12.;
file.write_value(b);
std::vector<double> v{1., 2., 3., 4., 6., 10., 15.};
file.write_vector(v);
iFile ifile("test.ndf");
double ib;
ifile.read_value(ib);
std::cout << ib << std::endl;
std::vector<double> iv;
ifile.read_vector(iv);
std::cout << "vector size: " << iv.size() << std::endl;
for(auto val : iv){
std::cout << val << " ";
}
std::cout << "last val: " << iv[iv.size() - 1] << std::endl;
} // <-- all local variables incl. file will be destroyed here
的 ofile file 寿命到结束 main.
wrote a value: 12 current offset: 8
wrote a value: 7 current offset: 16
wrote a value: 1 current offset: 24
wrote a value: 2 current offset: 32
wrote a value: 3 current offset: 40
wrote a value: 4 current offset: 48
wrote a value: 6 current offset: 56
wrote a value: 10 current offset: 64
wrote a value: 15 current offset: 72
value read: 12 current offset: 8
12
value read: 7 current offset: 16
size of vector: 7
value read: 1 current offset: 24
value read: 2 current offset: 32
value read: 3 current offset: 40
value read: 4 current offset: 48
value read: 6 current offset: 56
value read: 10 current offset: 64
value read: 10 current offset: 72
vector size: 7
1 2 3 4 6 10 10 last val: 10
现场演示在coliru上 (我在这里转载了OP的问题)。
由于内部流在分解器的 ofile在这之前，可能会有内部缓冲（即未刷新）的内容，尽管流会将它们报告为写入。
解决方法很简单。解决办法很简单： ofile file 必须要有限制。
的固定代码。
int main()
{
std::vector<double> v{1., 2., 3., 4., 6., 10., 15.};
{ // start new scope
oFile file("test.ndf");
double b = 12.;
file.write_value(b);
file.write_vector(v);
} // close scope -> destroy file (and b)
iFile ifile("test.ndf");
double ib;
ifile.read_value(ib);
std::cout << ib << std::endl;
std::vector<double> iv;
ifile.read_vector(iv);
std::cout << "vector size: " << iv.size() << std::endl;
for(auto val : iv){
std::cout << val << " ";
}
std::cout << "last val: " << iv[iv.size() - 1] << std::endl;
}
输出：
wrote a value: 12 current offset: 8
wrote a value: 7 current offset: 16
wrote a value: 1 current offset: 24
wrote a value: 2 current offset: 32
wrote a value: 3 current offset: 40
wrote a value: 4 current offset: 48
wrote a value: 6 current offset: 56
wrote a value: 10 current offset: 64
wrote a value: 15 current offset: 72
value read: 12 current offset: 8
12
value read: 7 current offset: 16
size of vector: 7
value read: 1 current offset: 24
value read: 2 current offset: 32
value read: 3 current offset: 40
value read: 4 current offset: 48
value read: 6 current offset: 56
value read: 10 current offset: 64
value read: 15 current offset: 72
vector size: 7
1 2 3 4 6 10 15 last val: 15
我不明白为什么它要写或读最后一个值2次？
这一点我也不清楚，直到我意识到我把太多的注意力集中在了 读取最后一次数值2次.
事实上，它不 - 只是未能读取最后一个值，并再次错误地报告前一个值。.
为了验证这一点，我在这里添加了一个最小的 "错误处理"。iFile::read_value:
template <typename T>
void read_value(T& v){
file.seekg(offset);
file.read(reinterpret_cast<char*> (&v), sizeof(T));
if (!file) std::cerr << "AARG! Input failed. :-(\n";
offset += sizeof(T);
std::cout << "value read: " << v << " current offset: " << offset << std::endl;
}
输出。
wrote a value: 12 current offset: 8
wrote a value: 7 current offset: 16
wrote a value: 1 current offset: 24
wrote a value: 2 current offset: 32
wrote a value: 3 current offset: 40
wrote a value: 4 current offset: 48
wrote a value: 6 current offset: 56
wrote a value: 10 current offset: 64
wrote a value: 15 current offset: 72
value read: 12 current offset: 8
12
value read: 7 current offset: 16
size of vector: 7
value read: 1 current offset: 24
value read: 2 current offset: 32
value read: 3 current offset: 40
value read: 4 current offset: 48
value read: 6 current offset: 56
value read: 10 current offset: 64
AARG! Input failed. :-(
value read: 10 current offset: 72
vector size: 7
1 2 3 4 6 10 10 last val: 10
当然，文件IO是可能因为各种原因而失败的。所以，应随时检查文件操作的成功率。