ECMGenerateExportHeader — Extra CMake Modules 6.29.0 documentation

# ECMGenerateExportHeader

This module provides the `ecm_generate_export_header` function for generating export macros for libraries with version-based control over visibility of and compiler warnings for deprecated API for the library user, as well as over excluding deprecated API and their implementation when building the library itself.

For preparing some values useful in the context it also provides a function `ecm_export_header_format_version`.

    ecm_generate_export_header(<library_target_name>
        VERSION <version>
        [BASE_NAME <base_name>]
        [GROUP_BASE_NAME <group_base_name>]
        [EXPORT_MACRO_NAME <export_macro_name>]
        [EXPORT_FILE_NAME <export_file_name>]
        [DEPRECATED_MACRO_NAME <deprecated_macro_name>]
        [NO_EXPORT_MACRO_NAME <no_export_macro_name>]
        [INCLUDE_GUARD_NAME <include_guard_name>]
        [STATIC_DEFINE <static_define>]
        [PREFIX_NAME <prefix_name>]
        [DEPRECATED_BASE_VERSION <deprecated_base_version>]
        [DEPRECATION_VERSIONS <deprecation_version> [<deprecation_version2> [...]]]
        [EXCLUDE_DEPRECATED_BEFORE_AND_AT <exclude_deprecated_before_and_at_version>]
        [DEPRECATED_ATTRIBUTE_TYPE <attribute_type>] #  Since 6.28
        [NO_BUILD_SET_DEPRECATED_WARNINGS_SINCE]
        [NO_DEFINITION_EXPORT_TO_BUILD_INTERFACE]
        [USE_VERSION_HEADER [<version_file_name>]] #  Since 5.106
        [VERSION_BASE_NAME <version_base_name>] #  Since 5.106
        [VERSION_MACRO_NAME <version_macro_name>] #  Since 5.106
        [CUSTOM_CONTENT_FROM_VARIABLE <variable>]
    )

`VERSION` specifies the version of the library, given in the format “<major>.<minor>.<patchlevel>”.

`GROUP_BASE_NAME` specifies the name to use for the macros defining library group default values. If set, this will generate code supporting `<group_base_name>_NO_DEPRECATED_WARNINGS`, `<group_base_name>_DISABLE_DEPRECATED_BEFORE_AND_AT`, `<group_base_name>_DEPRECATED_WARNINGS_SINCE` and `<group_base_name>_NO_DEPRECATED` (see below). If not set, the generated code will ignore any such macros.

`DEPRECATED_BASE_VERSION` specifies the default version before and at which deprecated API is disabled. Possible values are “0”, “CURRENT” (which resolves to <version>) and a version string in the format “<major>.<minor>.<patchlevel>”. The default is the value of “<exclude_deprecated_before_and_at_version>” if set, or “<major>.0.0”, with <major> taken from <version>.

`DEPRECATION_VERSIONS` specifies versions in “<major>.<minor>” format in which API was declared deprecated. Any version used with the generated macro `<prefix_name><base_name>_DEPRECATED_VERSION(major, minor, text)` or `<prefix_name><base_name>_DEPRECATED_VERSION_BELATED(major, minor, textmajor, textminor, text)` needs to be listed here, otherwise the macro will fail to work.

`EXCLUDE_DEPRECATED_BEFORE_AND_AT` specifies the version for which all API deprecated before and at should be excluded from the build completely. Possible values are “0” (default), “CURRENT” (which resolves to <version>) and a version string in the format “<major>.<minor>.<patchlevel>”.

`DEPRECATED_ATTRIBUTE_TYPE` specifies the type of attribute to use in the generated macros. Options are “STANDARD” (default when the minimum required version of ECM is 6.28 or newer) or “COMPILER” (default otherwise, legacy). “STANDARD” will result in the use of the C++ standard attribute `[[deprecated("text")]]`, where-as “COMPILER” will use the pre-C++14 compiler-specific annotations like `__attribute__ ((__deprecated__("text")))` or `__declspec(deprecated("text"))`, if available. Since 6.28.

Warning

The compiler-specific annotation attributes can be placed more freely in the signatures of declarations, the C++ standard `[[deprecated("text")]]` ones are restrained to the standard locations for attributes. So when switching from “COMPILER” to “STANDARD” it needs to be ensured all used deprecation macros are only at locations where attributes are possible. Also note that with “STANDARD” the special enumerator macros (see below) will not be available, as usually the normal deprecation macro can be used.

`NO_BUILD_SET_DEPRECATED_WARNINGS_SINCE` specifies that the definition `<prefix_name><uppercase_base_name>_DEPRECATED_WARNINGS_SINCE` will not be set for the library inside its own build, and thus will be defined by either explicit definition in the build system configuration or by the default value mechanism (see below). The default is that it is set for the build, to the version specified by `EXCLUDE_DEPRECATED_BEFORE_AND_AT`, so no deprecation warnings are done for any own deprecated API used in the library implementation itself.

`NO_DEFINITION_EXPORT_TO_BUILD_INTERFACE` specifies that the definition `<prefix_name><uppercase_base_name>_DISABLE_DEPRECATED_BEFORE_AND_AT` will not be set in the public interface of the library inside its own build, and the same for the definition `<prefix_name><uppercase_base_name>_DEPRECATED_WARNINGS_SINCE` (if not disabled by `NO_BUILD_SET_DEPRECATED_WARNINGS_SINCE` already). The default is that they are set, to the version specified by `EXCLUDE_DEPRECATED_BEFORE_AND_AT`, so e.g. test and examples part of the project automatically build against the full API included in the build and without any deprecation warnings for it.

`USE_VERSION_HEADER` defines whether a given header file `<version_file_name>` providing macros specifying the library version should be included in the generated header file. By default angle-brackets are used for the include statement. To generate includes with double quotes, add double quotes to the argument string (needs escaping), e.g. `\"version.h\"`. The macro from the included version header holding the library version is given as `<version_macro_name>` by the argument `VERSION_MACRO_NAME` and used in the generated code for calculating defaults. If not specified, the defaults for the version file name and the version macro are derived from `<version_base_name>` as passed with `VERSION_BASE_NAME`, which again defaults to `<base_name>` or otherwise `<library_target_name>`. The macro name defaults to `<uppercase_version_base_name>_VERSION`, the version file name to `<lowercase_version_base_name>_version.h`. Since 5.106.

`CUSTOM_CONTENT_FROM_VARIABLE` specifies the name of a variable whose content will be appended at the end of the generated file, before any final inclusion guard closing. Note that before 5.98 this was broken and would only append the string passed as argument value.

The function `ecm_generate_export_header` defines C++ preprocessor macros in the generated export header, some for use in the sources of the library the header is generated for, other for use by projects linking against the library.

The macros for use in the library C++ sources are these, next to those also defined by GenerateExportHeader:

`<prefix_name><uppercase_base_name>_DEPRECATED_VERSION(major, minor, text)`

  to use to conditionally set a `<prefix_name><uppercase_base_name>_DEPRECATED` macro for a class, struct or function (other elements to be supported in future versions), depending on the visibility macro flags set (see below)

`<prefix_name><uppercase_base_name>_DEPRECATED_VERSION_BELATED(major, minor, textmajor, textminor, text)`

  to use to conditionally set a `<prefix_name><uppercase_base_name>_DEPRECATED` macro for a class, struct or function (other elements to be supported in future versions), depending on the visibility macro flags set (see below), with `major` & `minor` applied for the logic and `textmajor` & `textminor` for the warnings message. Useful for retroactive tagging of API for the compiler without injecting the API into the compiler warning conditions of already released versions. Since 5.71.

`<prefix_name><uppercase_base_name>_ENUMERATOR_DEPRECATED_VERSION(major, minor, text)`

  to use to conditionally set a `<prefix_name><uppercase_base_name>_DEPRECATED` macro for an enumerator, depending on the warnings macro flags set (see below). In builds using C++14 standard or earlier, where enumerator attributes are not yet supported, the macro will always yield an empty string. With MSVC it is also always an empty string for now. Not available with `DEPRECATED_ATTRIBUTE_TYPE` set to “STANDARD”, there the normal macro can be usually used. Since 5.82.

`<prefix_name><uppercase_base_name>_ENUMERATOR_DEPRECATED_VERSION_BELATED(major, minor, textmajor, textminor, text)`

  to use to conditionally set a `<prefix_name><uppercase_base_name>_DEPRECATED` macro for an enumerator, depending on the warnings macro flags set (see below), with `major` & `minor` applied for the logic and `textmajor` & `textminor` for the warnings message. In builds using C++14 standard or earlier, where enumerator attributes are not yet supported, the macro will always yield an empty string. Useful for retroactive tagging of API for the compiler without injecting the API into the compiler warning conditions of already released versions. With MSVC it is also always an empty string for now. Not available with `DEPRECATED_ATTRIBUTE_TYPE` set to “STANDARD”, there the normal macro can be usually used. Since 5.82.

`<prefix_name><uppercase_base_name>_ENABLE_DEPRECATED_SINCE(major, minor)`

  evaluates to `TRUE` or `FALSE` depending on the visibility macro flags set (see below). To be used mainly with `#if`/`#endif` to mark sections of code which should be included depending on the visibility requested.

`<prefix_name><uppercase_base_name>_BUILD_DEPRECATED_SINCE(major, minor)`

  evaluates to `TRUE` or `FALSE` depending on the value of `EXCLUDE_DEPRECATED_BEFORE_AND_AT`. To be used mainly with `#if`/`#endif` to mark sections of two types of code: implementation code for deprecated API and declaration code of deprecated API which only may be disabled at build time of the library for BC reasons (e.g. virtual methods, see notes below).

`<prefix_name><uppercase_base_name>_EXCLUDE_DEPRECATED_BEFORE_AND_AT`

  holds the version used to exclude deprecated API at build time of the library.

The macros used to control visibility when building against the library are:

`<prefix_name><uppercase_base_name>_DISABLE_DEPRECATED_BEFORE_AND_AT`

  definition to set to a value in single hex number version notation (`0x<major><minor><patchlevel>`).

`<prefix_name><uppercase_base_name>_NO_DEPRECATED`

  flag to define to disable all deprecated API, being a shortcut for settings `<prefix_name><uppercase_base_name>_DISABLE_DEPRECATED_BEFORE_AND_AT` to the current version. If both are set, this flag overrules.

`<prefix_name><uppercase_base_name>_DEPRECATED_WARNINGS_SINCE`

  definition to set to a value in single hex number version notation (`0x<major><minor><patchlevel>`). Warnings will be only activated for API deprecated up to and including the version. If `<prefix_name><uppercase_base_name>_DISABLE_DEPRECATED_BEFORE_AND_AT` is set (directly or via the group default), it will default to that version, resulting in no warnings. Otherwise the default is the current version, resulting in warnings for all deprecated API.

`<prefix_name><uppercase_base_name>_NO_DEPRECATED_WARNINGS`

  flag to define to disable all deprecation warnings, being a shortcut for setting `<prefix_name><uppercase_base_name>_DEPRECATED_WARNINGS_SINCE` to “0”. If both are set, this flag overrules.

When the `GROUP_BASE_NAME` has been used, the same macros but with the given `<group_base_name>` prefix are available to define the defaults of these macros, if not explicitly set.

Warning

The tricks applied here for hiding deprecated API to the compiler when building against a library do not work for all deprecated API:

- virtual methods need to stay visible to the compiler to build proper virtual method tables for subclasses

- enumerators from enums cannot be simply removed, as this changes auto values of following enumerators, also can poke holes in enumerator series used as index into tables

In such cases the API can be only “hidden” at build time of the library, itself, by generated hard coded macro settings, using `<prefix_name><uppercase_base_name>_BUILD_DEPRECATED_SINCE(major, minor)`.

Examples:

Preparing a library “Foo” created by target “foo”, which is part of a group of libraries “Bar”, where some API of “Foo” got deprecated at versions 5.0 & 5.12:

    ecm_generate_export_header(foo
        GROUP_BASE_NAME BAR
        VERSION ${FOO_VERSION}
        DEPRECATION_VERSIONS 5.0 5.12
    )

In the library “Foo” sources in the headers the API would be prepared like this, using the generated macros `FOO_ENABLE_DEPRECATED_SINCE` and `FOO_DEPRECATED_VERSION`:

    #include <foo_export.h>

    #if FOO_ENABLE_DEPRECATED_SINCE(5, 0)
    /**
      * @deprecated Since 5.0
      */
    FOO_EXPORT
    FOO_DEPRECATED_VERSION(5, 0, "Use doFoo2()")
    void doFoo();
    #endif

    #if FOO_ENABLE_DEPRECATED_SINCE(5, 12)
    /**
      * @deprecated Since 5.12
      */
    FOO_EXPORT
    FOO_DEPRECATED_VERSION(5, 12, "Use doBar2()")
    void doBar();
    #endif

Projects linking against the “Foo” library can control which part of its deprecated API should be hidden to the compiler by adding a definition using the `FOO_DISABLE_DEPRECATED_BEFORE_AND_AT` macro variable set to the desired value (in version hex number notation):

    add_definitions(-DFOO_DISABLE_DEPRECATED_BEFORE_AND_AT=0x050000)

Or using the macro variable of the group:

    add_definitions(-DBAR_DISABLE_DEPRECATED_BEFORE_AND_AT=0x050000)

If both are specified, `FOO_DISABLE_DEPRECATED_BEFORE_AND_AT` will take precedence.

To build a variant of a library with some deprecated API completely left out from the build, not only optionally invisible to consumers, one uses the `EXCLUDE_DEPRECATED_BEFORE_AND_AT` parameter. This is best combined with a cached CMake variable.

    set(EXCLUDE_DEPRECATED_BEFORE_AND_AT 0 CACHE STRING "Control the range of deprecated API excluded from the build [default=0].")

    ecm_generate_export_header(foo
        VERSION ${FOO_VERSION}
        EXCLUDE_DEPRECATED_BEFORE_AND_AT ${EXCLUDE_DEPRECATED_BEFORE_AND_AT}
        DEPRECATION_VERSIONS 5.0 5.12
    )

The macros used in the headers for library consumers are reused for disabling the API excluded in the build of the library. For disabling the implementation of that API as well as for disabling deprecated API which only can be disabled at build time of the library for BC reasons, one uses the generated macro `FOO_BUILD_DEPRECATED_SINCE`, like this:

    #include <foo_export.h>

    enum Bars {
        One,
    #if FOO_BUILD_DEPRECATED_SINCE(5, 0)
        Two FOO_ENUMERATOR_DEPRECATED_VERSION(5, 0, "Use Three"), // macro available since 5.82
    #endif
        Three,
    };

    #if FOO_ENABLE_DEPRECATED_SINCE(5, 0)
    /**
      * @deprecated Since 5.0
      */
    FOO_EXPORT
    FOO_DEPRECATED_VERSION(5, 0, "Use doFoo2()")
    void doFoo();
    #endif

    #if FOO_ENABLE_DEPRECATED_SINCE(5, 12)
    /**
      * @deprecated Since 5.12
      */
    FOO_EXPORT
    FOO_DEPRECATED_VERSION(5, 12, "Use doBar2()")
    void doBar();
    #endif

    class FOO_EXPORT Foo {
    public:
    #if FOO_BUILD_DEPRECATED_SINCE(5, 0)
        /**
          * @deprecated Since 5.0
          */
        FOO_DEPRECATED_VERSION(5, 0, "Feature removed")
        virtual void doWhat();
    #endif
    };

    #if FOO_BUILD_DEPRECATED_SINCE(5, 0)
    void doFoo()
    {
        // [...]
    }
    #endif

    #if FOO_BUILD_DEPRECATED_SINCE(5, 12)
    void doBar()
    {
        // [...]
    }
    #endif

    #if FOO_BUILD_DEPRECATED_SINCE(5, 0)
    void Foo::doWhat()
    {
        // [...]
    }
    #endif

So e.g. if `EXCLUDE_DEPRECATED_BEFORE_AND_AT` is set to “5.0.0”, the enumerator `Two` as well as the methods `::doFoo()` and `Foo::doWhat()` will be not available to library consumers. The methods will not have been compiled into the library binary, and the declarations will be hidden to the compiler, `FOO_DISABLE_DEPRECATED_BEFORE_AND_AT` also cannot be used to reactivate them.

When using the `NO_DEFINITION_EXPORT_TO_BUILD_INTERFACE` and the project for the “Foo” library includes also tests and examples linking against the library and using deprecated API (like tests covering it), one better explicitly sets `FOO_DISABLE_DEPRECATED_BEFORE_AND_AT` for those targets to the version before and at which all deprecated API has been excluded from the build. Even more when building against other libraries from the same group “Bar” and disabling some deprecated API of those libraries using the group macro `BAR_DISABLE_DEPRECATED_BEFORE_AND_AT`, which also works as default for `FOO_DISABLE_DEPRECATED_BEFORE_AND_AT`.

To get the hex number style value the helper macro `ecm_export_header_format_version()` will be used:

    set(EXCLUDE_DEPRECATED_BEFORE_AND_AT 0 CACHE STRING "Control what part of deprecated API is excluded from build [default=0].")

    ecm_generate_export_header(foo
        VERSION ${FOO_VERSION}
        GROUP_BASE_NAME BAR
        EXCLUDE_DEPRECATED_BEFORE_AND_AT ${EXCLUDE_DEPRECATED_BEFORE_AND_AT}
        NO_DEFINITION_EXPORT_TO_BUILD_INTERFACE
        DEPRECATION_VERSIONS 5.0 5.12
    )

    ecm_export_header_format_version(${EXCLUDE_DEPRECATED_BEFORE_AND_AT}
        CURRENT_VERSION ${FOO_VERSION}
        HEXNUMBER_VAR foo_no_deprecated_before_and_at
    )

    # disable all deprecated API up to 5.9.0 from all other libs of group "BAR" that we use ourselves
    add_definitions(-DBAR_DISABLE_DEPRECATED_BEFORE_AND_AT=0x050900)

    add_executable(app app.cpp)
    target_link_libraries(app foo)
    target_compile_definitions(app
         PRIVATE "FOO_DISABLE_DEPRECATED_BEFORE_AND_AT=${foo_no_deprecated_before_and_at}")

Since 5.64.0.