关于CMake自定义find_package模块的两处技术疑问

Hey there! Let's work through your two questions about the custom find_package() module from the CMake Cookbook examples:

1. The _zmqver_EXTRACT Function's Version Value Issue #

You’re absolutely right to question this — that code as described would indeed always set the version to 0, which is clearly a bug (either in the example you’re looking at, or maybe a typo in how it was presented).

Normally, the _zmqver_EXTRACT function is meant to pull version information from ZeroMQ's header files (like zmq.h) using regex matching. Here’s what the correct flow should look like:

• Read the content of the ZeroMQ header file into a variable.
• Use string(REGEX MATCH) to find lines like #define ZMQ_VERSION_MAJOR 4, capturing the numeric value into CMAKE_MATCH_1.
• Then pass that captured value back to the parent scope using set(${_ZeroMQ_VER_OUTPUT} "${CMAKE_MATCH_1}" PARENT_SCOPE).

If the code explicitly sets CMAKE_MATCH_1 to 0 before that set call, it’s overriding the regex match result. Your intuition that the version info should end up in a variable like ZeroMQ_ver is spot-on — that’s the intended behavior.

Here’s a quick corrected snippet of what that function should look like:

function(_zmqver_EXTRACT _header _var)
  file(READ "${_header}" _zmq_header_content)
  string(REGEX MATCH "#define ZMQ_VERSION_MAJOR ([0-9]+)" _dummy "${_zmq_header_content}")
  set(${_var} "${CMAKE_MATCH_1}" PARENT_SCOPE)
endfunction()

2. What’s the ZeroMQ_FIND_VERSION_COUNT Variable For? #

Great question — this is a CMake-automatic variable, meaning you don’t need to define it yourself. When you call find_package(ZeroMQ) with a version specifier (like find_package(ZeroMQ 4.3.2)), CMake automatically sets several helper variables, including:

• ZeroMQ_FIND_VERSION: The full version string you specified (e.g., 4.3.2)
• ZeroMQ_FIND_VERSION_MAJOR: The major version (4)
• ZeroMQ_FIND_VERSION_MINOR: The minor version (3)
• ZeroMQ_FIND_VERSION_PATCH: The patch version (2)
• ZeroMQ_FIND_VERSION_COUNT: The number of components in the specified version (3 in this case; if you just specified 4, it would be 1)

This variable is useful in your find module to adjust how strict your version check is. For example:

• If ZeroMQ_FIND_VERSION_COUNT is 1, you only need to verify that the found ZeroMQ major version matches what the user requested.
• If it’s 3, you need to check that major, minor, and patch versions all meet or exceed the requested version.

It’s a helper variable to make your version-checking logic more flexible without having to manually parse the version string yourself.

内容的提问来源于stack exchange，提问作者Sean