Specifying a violation handler for contracts

Support for contract based programming in C++
was adopted ino the C++20 working draft in Rapperswil. One part of this language feature is this notion of a violation handler which will be invoked when a contract is violated.

Herb Sutter's trip report states that:

You get to install your own violation handler and ship a release build with the option of turning on enforcement at run time.

But the wording in [dcl.attr.contract] that this paper added says:

The violation handler of a program is a function of type “noexcept_opt
function of (lvalue reference to const std​::​contract_­violation) returning void”, and is specified in an implementation-defined manner. [...] There should be no programmatic way of setting or modifying the violation handler. It is implementation-defined how the violation handler is established for a program and how the std​::​contract_­violation ([support.contract.cviol]) argument value is set, except as specified below.

noexcept

const std​::​contract_­violation

void

std​::​contract_­violation

This is very unclear to me. How might an implementation allow me to set my own violation handler, in a non-programmatic way? What will I have to do on gcc, clang, and msvc?

1 Answer
1

How might an implementation allow me to set my own violation handler, in a non-programmatic way?

That's for implementations to determine, but I rather suspect it would be a command line parameter of some sort. You'd name a function, and the compiler/linker would make that the violation handler. And if not that, then they would probably pick some specific function name that you implement.

The point here is that the function being used is static, from the perspective of the C++ abstract model. When the compiler starts, it knows exactly which function will be called, much like the call by the system to main, as well as the part of the program that handles main's return values.

main

main

terminate

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