Traits are properties of a requires clause. They determine how various parts of a dependency are treated and propagated by Conan. Values for traits are usually computed by Conan based on the dependency’s package_type, but can also be specified manually.
A good introduction to traits is provided in the Advanced Dependencies Model in Conan 2.0 presentation.
In the example below
libs are traits.
self.requires("math/1.0", headers=True, libs=True)
Indicates that there are headers that are going to be
#included from this package at
compile time. The dependency will be in the host context.
The dependency contains some library or artifact that will be used at link time of the
consumer. This trait will typically be
True for direct shared and static libraries,
but could be false for indirect static libraries that are consumed via a shared library.
The dependency will be in the host context.
This dependency is a build tool, an application or executable, like cmake, that is used exclusively at build time. It is not linked/embedded into binaries, and will be in the build context.
This dependency contains some executables, either apps or shared libraries that need to be
available to execute (typically in the path, or other system env-vars). This trait can be
build=False, in that case, the package will contain some executables that
can run in the host system when installing it, typically like an end-user application.
This trait can be
build=True, the package will contain executables that
will run in the build context, typically while being used to build other packages.
require will be propagated downstream, even if it doesn’t propagate
run traits. Requirements that propagate downstream can cause version
conflicts. This is typically
True, because in most cases, having 2 different versions of
the same library in the same dependency graph is at least complicated, if not directly
violating ODR or causing linking errors. It can be set to
False in advanced scenarios,
when we want to use different versions of the same package during the build.
True the headers of the dependency will be visible downstream.
True the libraries to link with of the dependency will be visible downstream.
This requirement is a test library or framework, like Catch2 or gtest. It is mostly a library that needs to be included and linked, but that will not be propagated downstream.
If the recipe wants to specify how the dependency version affects the current package
package_id, can be directly specified here.
While it could be also done in the
package_id() method, it seems simpler to be able to
specify it in the
requires while avoiding some ambiguities.
# We set the package_id_mode so it is part of the package_id self.tool_requires("tool/1.1.1", package_id_mode="minor_mode")
Which would be equivalent to:
def package_id(self): self.info.requires["tool"].minor_mode()
requires will force its version in the dependency graph upstream, overriding
other existing versions even of transitive dependencies, and also solving potential
existing conflicts. The downstream consumer’s
force traits always have higher priority.
The same as the
force trait, but not adding a
direct dependency. If there is no
transitive dependency to override, this
require will be discarded. This trait only
exists at the time of defining a
requires, but it will not exist as an actual
requires once the graph is fully evaluated
override traits to solve conflicts are not recommended as a general versioning
solution, just as a temporary workaround to solve a version conflict. Its usage should be avoided
whenever possible, and updating versions or version ranges in the graph to avoid the conflicts without
overrides and forces is the recommended approach.
If the dependency is a direct one, that is, it has explicitly been declared by the current recipe, or if it is a transitive one.
package_type trait inferring¶
Some traits are automatically inferred based on the value of the
package_type if not explicitly set by the recipe.
Additionally, some additional traits are inferred on top of the above mentioned based on the
package_type of the dependant:
Default traits for each kind of requires¶
Each kind of requires sets some additional traits by default on top of the ones stated in the last section. Those are: