CppCon 2018: Michael Price “Concepts and Contracts: When, What, and How”

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Challenge: Write a library function template that takes exactly one positive, integral parameter.

There are many different ways to implement this function template in standard C++. The future additions of Concepts and Contracts will provide even more, possibly simpler, implementations as well. We will review the current status of both of these important new features, conduct a brief overview of what they can do, and then examine how they can work together towards an interface specification nirvana.
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Michael Price, Synopsys, Inc.
Senior Software Engineer

Michael Price has developed and taught C++ for more than a decade and has been an active participant in WG21 since 2014, allocating most of his committee-time to EWG and the Reflection Study Group. He professes interest in making C++ "safer" to use through language and library design, automation, testing, and education. His past work includes enterprise-grade screen sharing applications, ABI-safe C++ component design, network protocol design, software development utilities, large-scale build and test automation, C/C++ compiler front-end development for static analysis tools, and a smidge of management experience.
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[connorhorman]

I would say that unsigned char and signed char are integer types (as frequently I use int8_t and uint8_t which are commonly (un)signed char, as integers). Also wchar_t, char16_t, and char32_t are all integral, but they are probably not integer types.

[cmdlp]

There should be a way to check if an expression is defined. It would be false, if the compiler knows that it is undefined; otherwise it would return true:


static_assert(defined(1/0) == false);
static_assert(defined(1/1) == true);

int i;
// initialize i to any value including 0
[[assert: defined(1/i) == (i!=0)]];


In this case, defined is dependent on values at runtime and so it is calculated at runtime, the expression inside is never evaluated. There might be something like always_defined() to get a compile time constant:


int i;
int* p = &i;
int* q = (int*)malloc(sizeof(int)); // can be nullptr


// -> ERROR


This would also be used for checking if the iterators of a range are from the same container:


[[expects: defined(std::distance(first, last))]]; // the distance is only defined for two iterators from the same container.


It should not be a problem to be implemented in a compiler, because the compiler uses undefined behavior anyway to optimize the code.

[gyuntist]

His presentation material was disappeared from his Github repository.

[YourCRTube]

Contracts should be checked at evaluation time, which means runtime or compiletime, everything else is unintuitive.