Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of Boost. Click here to view this page for the latest version.

File

The File concept abstracts access to files in the underlying file system. To support other platform interfaces, users may author their own File types which meet these requirements.

Associated Types

Requirements

In this table:

F is a File type
f is an instance of F
p is a value of type char const* which points to a null terminated utf-8 encoded string.
m is an instance of file_mode
n is a number of bytes, convertible to std::size_t
o is a byte offset in the file, convertible to std::uint64_t
b is any non-const pointer to memory
c is any possibly-const pointer to memory
ec is a reference of type error_code

Table 1.42. Valid expressions

Expression	Type	Semantics, Pre/Post-conditions
Operation	Return Type	Semantics, Pre/Post-conditions
`F()`		Default constructable
`f.~F()`		Destructible. If `f` refers to an open file, it is first closed as if by a call to `close` with the error ignored.
`f.is_open()`	`bool`	Returns `true` if `f` refers to an open file, `false` otherwise.
`f.close(ec)`		If `f` refers to an open file, this function attempts to close the file. Regardless of whether an error occurs or not, a subsequent call to `f.is_open()` will return `false`. The function will ensure that `!ec` is `true` if there was no error or set to the appropriate error code if an error occurred.
`f.open(p,m,ec)`		Attempts to open the file at the path specified by `p` with the mode specified by `m`. Upon success, a subsequent call to `f.is_open()` will return `true`. If `f` refers to an open file, it is first closed as if by a call to `close` with the error ignored. The function will ensure that `!ec` is `true` if there was no error or set to the appropriate error code if an error occurred.
`f.size(ec)`	`std::uint64_t`	If `f` refers to an open file, this function attempts to determine the file size and return its value. If `f` does not refer to an open file, the function will set `ec` to `errc::invalid_argument` and return 0. The function will ensure that `!ec` is `true` if there was no error or set to the appropriate error code if an error occurred.
`f.pos(ec)`	`std::uint64_t`	If `f` refers to an open file, this function attempts to determine the current file offset and return it. If `f` does not refer to an open file, the function will set `ec` to `errc::invalid_argument` and return 0. The function will ensure that `!ec` is `true` if there was no error or set to the appropriate error code if an error occurred.
`f.seek(o,ec)`		Attempts to reposition the current file offset to the value `o`, which represents a byte offset relative to the beginning of the file. If `f` does not refer to an open file, the function will set `ec` to `errc::invalid_argument` and return immediately. The function will ensure that `!ec` is `true` if there was no error or set to the appropriate error code if an error occurred.
`f.read(b,n,ec)`	`std::size_t`	Attempts to read `n` bytes starting at the current file offset from the open file referred to by `f`. Bytes read are stored in the memory buffer at address `b` which must be at least `n` bytes in size. The function advances the file offset by the amount read, and returns the number of bytes actually read, which may be less than `n`. If `f` does not refer to an open file, the function will set `ec` to `errc::invalid_argument` and return immediately. The function will ensure that `!ec` is `true` if there was no error or set to the appropriate error code if an error occurred.
`f.write(c,n,ec)`	`std::size_t`	Attempts to write `n` bytes from the buffer pointed to by `c` to the current file offset of the open file referred to by `f`. The memory buffer at `c` must point to storage of at least `n` bytes meant to be copied to the file. The function advances the file offset by the amount written, and returns the number of bytes actually written, which may be less than `n`. If `f` does not refer to an open file, the function will set `ec` to `errc::invalid_argument` and return immediately. The function will ensure that `!ec` is `true` if there was no error or set to the appropriate error code if an error occurred.

Exemplar

struct File
{
    /** Default constructor

        There is no open file initially.
    */
    File();

    /** Destructor

        If the file is open it is first closed.
    */
    ~File();

    /// Returns `true` if the file is open
    bool
    is_open() const;

    /// Close the file if open
    void
    close(error_code& ec);

    /// Open a file at the given path with the specified mode
    void
    open(char const* path, file_mode mode, error_code& ec);

    /// Return the size of the open file
    std::uint64_t
    size(error_code& ec) const;

    /// Return the current position in the open file
    std::uint64_t
    pos(error_code& ec) const;

    /// Adjust the current position in the open file
    void
    seek(std::uint64_t offset, error_code& ec);

    /// Read from the open file
    std::size_t
    read(void* buffer, std::size_t n, error_code& ec) const;

    /// Write to the open file
    std::size_t
    write(void const* buffer, std::size_t n, error_code& ec);
};

Models