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type: table cell
Returns `true` if `f` refers to an open file, `false` otherwise.
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Key English Chinese (Simplified Han script) Actions
In this table:
在下表中：
* `F` is a [*File] type
* `f` is an instance of `F`
* `p` is a value of type `char const*` which points to a null
* `F` 是一个 [*File] 类型。
* `f` 是 `F` 的一个实例。
* `p` 是一个 `char const*` 类型的值，指向一个空字符（null）
* `m` is an instance of [link beast.ref.boost__beast__file_mode `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 [link beast.ref.boost__beast__error_code `error_code`]
* `m` 是一个 [link beast.ref.boost__beast__file_mode `file_mode`] 的实例。
* `n` 是字节数，可转换为 `std::size_t` 类型。
* `o` 是文件中的字节偏移量，可转换为 `std::uint64_t` 类型。
* `b` 是指向内存的任意非常量指针。
* `c` 是指向内存的任意可能为常量的指针。
* `ec` 是一个 [link beast.ref.boost__beast__error_code `error_code`] 类型的引用。
Valid expressions
有效表达式
Expression
表达式
Type
类型
34
Semantics, Pre/Post-conditions
 
Operation
操作
Return Type
返回类型
`F()`
`F()`
Default constructable
默认构造（Default constructible）
`f.~F()`
`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.
可析构。若 `f` 引用一个打开的文件，则先将其关闭，其行为等同于调用 `close`，且忽略可能产生的错误。
`f.is_open()`
`f.is_open()`
`bool`
`bool`
Returns `true` if `f` refers to an open file, `false` otherwise.
若 `f` 引用一个打开的文件，则返回 `true`，否则返回 `false`。
`f.close(ec)`
`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.
若 `f` 引用一个打开的文件，此函数将尝试将其关闭。无论关闭过程中是否发生错误，后续对 `f.is_open()` 的调用均会返回 `false`。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。
`f.open(p,m,ec)`
`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.
该函数尝试以 `m` 指定的模式打开 `p` 指定的路径对应的文件。若 `f` 引用一个打开的文件，则先将其关闭，其行为等同于调用 `close`，且忽略可能产生的错误。若打开成功，后续对 `f.is_open()` 的调用均会返回 `true`。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。
`f.size(ec)`
`f.size(ec)`
34
`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.
若 `f` 引用一个打开的文件，该函数将尝试获取并返回其文件大小。若 `f` 未引用一个打开的文件，该函数会将 `ec` 设置为 `errc::invalid_argument` 并返回 0。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。
`f.pos(ec)`
`f.pos(ec)`
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.
若 `f` 引用一个打开的文件，该函数将尝试获取并返回当前文件偏移量。若 `f` 未引用一个打开的文件，该函数会将 `ec` 设置为 `errc::invalid_argument` 并返回 0。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。
`f.seek(o,ec)`
`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.
该函数尝试将当前文件偏移量重新定位到 `o`，该值表示相对于文件起始位置的字节偏移量。若 `f` 未引用一个打开的文件，该函数会将 `ec` 设置为 `errc::invalid_argument` 并立即返回。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。
`f.read(b,n,ec)`
`f.read(b,n,ec)`
34
`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. If an end-of-file condition is detected prior to reading any bytes, the function will ensure that `!ec` is `true` and the return value shall be 0.
该函数尝试从 `f` 引用的打开文件中，自当前文件偏移量处读取 `n` 个字节。读取的字节将存储到地址 `b` 处的内存缓冲区中，该缓冲区的大小必须至少为 `n` 个字节。该函数会将文件偏移量推进已读取的字节数，并返回实际读取的字节数，该值可能小于 `n`。若 `f` 未引用一个打开的文件，该函数会将 `ec` 设置为 `errc::invalid_argument` 并立即返回。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。若在读取任何字节之前检测到文件结束条件，该函数将确保 `!ec` 为 `true`，且返回值为 0。
`f.write(c,n,ec)`
`f.write(c,n,ec)`
Key English Chinese (Simplified Han script) Actions
Exemplar
示例
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.
该函数尝试将 `c` 指向的缓冲区中的 `n` 个字节写入 `f` 引用的打开文件的当前文件偏移量处。`c` 处的内存缓冲区必须指向大小至少为 `n` 个字节的存储，用于复制到文件。该函数会将文件偏移量推进已写入的字节数，并返回实际写入的字节数，该值可能小于 `n`。若 `f` 未引用一个打开的文件，该函数会将 `ec` 设置为 `errc::invalid_argument` 并立即返回。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。
Type
类型
* [link beast.ref.boost__beast__file_mode `file_mode`]
* [link beast.ref.boost__beast__is_file `is_file`]
* [link beast.ref.boost__beast__file_mode `file_mode`]
* [link beast.ref.boost__beast__is_file `is_file`]
Operation
操作
Default constructable
默认构造（Default constructible）
Destructible. If `f` refers to an open file, it is first closed as if by a call to `close` with the error ignored.
可析构。若 `f` 引用一个打开的文件，则先将其关闭，其行为等同于调用 `close`，且忽略可能产生的错误。
Expression
表达式
Associated Types
关联类型
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.
[*File] 概念抽象了对底层文件系统中文件的访问。为了支持其他平台接口，用户可以编写满足这些要求的自有 [*File] 类型。
Requirements
要求
In this table:
在下表中：
* `F` is a [*File] type
* `f` is an instance of `F`
* `p` is a value of type `char const*` which points to a null
* `F` 是一个 [*File] 类型。
* `f` 是 `F` 的一个实例。
* `p` 是一个 `char const*` 类型的值，指向一个空字符（null）
`f.~F()`
`f.~F()`
`f.is_open()`
`f.is_open()`
`bool`
`bool`
Returns `true` if `f` refers to an open file, `false` otherwise.
若 `f` 引用一个打开的文件，则返回 `true`，否则返回 `false`。
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.
若 `f` 引用一个打开的文件，此函数将尝试将其关闭。无论关闭过程中是否发生错误，后续对 `f.is_open()` 的调用均会返回 `false`。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。
`f.read(b,n,ec)`
`f.read(b,n,ec)`
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. If an end-of-file condition is detected prior to reading any bytes, the function will ensure that `!ec` is `true` and the return value shall be 0.
该函数尝试从 `f` 引用的打开文件中，自当前文件偏移量处读取 `n` 个字节。读取的字节将存储到地址 `b` 处的内存缓冲区中，该缓冲区的大小必须至少为 `n` 个字节。该函数会将文件偏移量推进已读取的字节数，并返回实际读取的字节数，该值可能小于 `n`。若 `f` 未引用一个打开的文件，该函数会将 `ec` 设置为 `errc::invalid_argument` 并立即返回。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。若在读取任何字节之前检测到文件结束条件，该函数将确保 `!ec` 为 `true`，且返回值为 0。
`f.write(c,n,ec)`
`f.write(c,n,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.
该函数尝试以 `m` 指定的模式打开 `p` 指定的路径对应的文件。若 `f` 引用一个打开的文件，则先将其关闭，其行为等同于调用 `close`，且忽略可能产生的错误。若打开成功，后续对 `f.is_open()` 的调用均会返回 `true`。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。
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.
若 `f` 引用一个打开的文件，该函数将尝试获取并返回其文件大小。若 `f` 未引用一个打开的文件，该函数会将 `ec` 设置为 `errc::invalid_argument` 并返回 0。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。
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.
若 `f` 引用一个打开的文件，该函数将尝试获取并返回当前文件偏移量。若 `f` 未引用一个打开的文件，该函数会将 `ec` 设置为 `errc::invalid_argument` 并返回 0。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。
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.
该函数尝试将当前文件偏移量重新定位到 `o`，该值表示相对于文件起始位置的字节偏移量。若 `f` 未引用一个打开的文件，该函数会将 `ec` 设置为 `errc::invalid_argument` 并立即返回。同时，该函数会根据实际执行情况设置错误代码：若未发生错误，则确保 `!ec` 为 `true`；若发生错误，则将其设置为相应的错误代码。
34
Semantics, Pre/Post-conditions
 
34
`std::size_t`
 
34
`std::uint64_t`
 

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User avatar Bstran

Translation changed
Returns `true` if `f` refers to an open file, `false` otherwise.
Returns `true` if `f` refers to an open file, `false` otherwise.若 `f` 引用一个打开的文件，则返回 `true`，否则返回 `false`。
06/19/2026
User avatar admin

String added in the repository
Returns `true` if `f` refers to an open file, `false` otherwise.
Returns `true` if `f` refers to an open file, `false` otherwise.
06/05/2026
Browse all string changes

Glossary

English Chinese (Simplified Han script)
No related strings found in the glossary.

String information

Source string description
type: table cell
Unit identifier
58596
Source string location
string ID 34
String age
06/05/2026
Last updated
06/26/2026
Source string age
06/05/2026
Translation file QuickBook file
doc/qbk/07_concepts/File_zh_Hans.qbk, string 21