ecl_concepts Documentation

ecl_concepts

Introduces a compile time concept checking mechanism that can be used most commonly to check for required functionality when passing template arguments.

• Homepage: http://wiki.ros.org/ecl_concepts

Embedded Control Library

One of the problem with template usage occurs occurs in the following scenario. For templatised functions and classes, the input template arguments usually have little or no restriction on the class that can be used, e.g.

template <typename T>
void f(T input) {
    // use some aspect of T's interface
}

Of course the compile will fail if you try to use an input type, T, that doesn't have the specialised interface you want, but the compile time error message is often very verbose and its difficult to find the error. An idea which is merging into the c++ mainstream as well as that used by the boost concept checking library is to characterise common behaviours into Concepts. A simplified version of this is utilised here.

Compiling & Linking

Include the following at the top of any translation unit that requires compilation of signals and/or slots.

#include <ecl/concepts.hpp>

// The concept definition/validation classes.
using ecl::BluePrintConcept;
using ecl::ContainerConcept;
using ecl::SignedByteContainerConcept;
using ecl::UnsignedByteContainerConcept;
using ecl::NullaryFunctionConcept;
using ecl::StreamConcept;
using ecl::InputCharDeviceConcept;
using ecl::OutputCharDeviceConcept;
using ecl::InputOutputCharDeviceConcept;
using ecl::InputByteDeviceConcept;
using ecl::OutputByteDeviceConcept;
using ecl::InputOutputByteDeviceConcept;

Since it is a collection of macros and template functions/classes, no linking is required if you are only utilising this functionality.

Usage

Usage is fairly simple. There are two steps:

• Concept Test : must be hand crafted to check all the characteristic behaviours of a concept.
• Concept Check : whenever required, call the concept check which runs the concept test to verify compliance.

Concept Test

If you are using an existing concept (see below), skip through to the concept check. Otherwise:

• Decide on a name for the concept.
• Create a templatised class of that name with a single arbitrary template argument (the class to be tested).
• Hand craft the testing function within that class using compile_time_concept_test().

The example here is the same as that used in src/tests/concepts.cpp:

template <typename T>
class MyConcept {
    public:
            compile_time_concept_test(MyConcept) {
                T t;
                t.apply();  // This is the characteristic function required by this concept.
            }
};

Hand-crafting the concept testing function will often require a little care to ensure the concept is appropriately covered.

Concept Check

Once you have a test in place, you only need to insert compile_time_concept_check() calls wherever you need them. Keep in mind, the sole purpose of these is to provide convenient error logs when something goes wrong.

class A { // This class has the MyConcept functionality.
    public:
        // The following function characterises the MyConcept concept.
        // Comment this line and any MyConcept concept assertion
        // check will fail in compile time.
        void apply() {};
};

int main() {
    compile_time_concept_check(MyConcept<A>); // Checks A for compliance to the MyConcept concept.
    return 0;
}

Existing Concepts

• BluePrints - checks for blueprint functionality as defined in ecl_utilities.
• Containers - also bytecontainer, simple checks for basic prerequisite container methods.
• NullaryFunctions - function objects which require no arguments in execution.
• Streams - any class with basic char streaming capabilities.
• Devices - device functionalities compatible to those in ecl_devices.

Examples

• src/test/concepts.cpp

ChangeLog

    - @ref changelogConcepts "ChangeLog"