C++文件处理打开

Question

while (file.read((char *)&vg, sizeof(vg))) what is significance of 'char' over here? I do not know the why do we use 'char' while reading text from 'dat' file in c++ programming.

Answer 1

I try to answer this even w/oa MCVE , assuming that file is a std::istream (including a class derived from that).

The member std::istream::read(char_type*, std::streamsize) takes as first argument a std::istream::char_type* = char* . The C-style syntax

(char*)(ptr)

casts ptr to a pointer to char . In this case, the address of vg is reinterpreted as the address to an array of char . This cast is necessary so that read() can be called. A char is used here to hold a byte: the code reads from file and writes the object vg byte by byte.

Answer 2

Assuming file is a std::fstream , that function read expects a char* , which means "a buffer of bytes into which I can read the value of the bytes from the file".

You didn't show a proper testcase, but we must assume that vg is not a buffer of bytes, but actually some object (like, perhaps, an integer). By casting a VgsType* to char* , you allow the read function to continue as if it were operating on the underlying bytes that make up vg .

Note that sometimes this is logically unsafe; it depends on what vg is. However, the actual cast to char* is valid because the standard specifically permits it as a way to inspect the byte representation of an object.

Answer 3

The following syntax (char*) it's a C style cast to char* . So it's taking the address of vg by doing &vg and converting it to a char* . The read function must need an argument of type char* or a type that can be implicitly convertible to char* .

It is generally better to use static_cast instead. Also I would try to read from file without needing to cast as general practice(as not all types can be casted to other types), that should be possible using the standard library streams.