configure_file路径疑惑

configure_file(<input> <output>
               [COPYONLY] [ESCAPE_QUOTES] [@ONLY]
               [NEWLINE_STYLE [UNIX|DOS|WIN32|LF|CRLF] ])

我遇到的是

configure_file(config/config.in ${CMAKE_SOURCE_DIR}/config/config.out)

疑惑在于config.in为什么不需要加路径，后来从cmake帮助文档中找到答案https://cmake.org/cmake/help/v3.2/command/configure_file.html

The arguments are:

<input>

Path to the input file. A relative path is treated with respect to the value of CMAKE_CURRENT_SOURCE_DIR. The input path must be a file, not a directory.因此默认可以不加之前的路径

<output>

Path to the output file or directory. A relative path is treated with respect to the value of CMAKE_CURRENT_BINARY_DIR. If the path names an existing directory the output file is placed in that directory with the same file name as the input file.