Contracts are a new feature that is being designed for the upcoming version of C++. If you want to know more about how this feature works in general, please check out our Contracts Working Paper,
P2900.
SG21 (The C++ Contracts Study Group in WG21) is requesting the opinion of SG22 (The C/C++ Liaison Group) on which candidate syntax for C++ Contracts would make it easier to adopt Contracts also for the C standard, ideally in a way that offers some useful C/C++ compatibility.
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The proposed new syntactic constructions are preconditions and postconditions (both of which go on function declarations) and assertions (which are statements or expressions inside a function body). Postconditions can optionally name the return value of a function (user-defined identifier before the colon). The two candidate syntax proposals for C++ Contracts are:
P2935R3, aka the "attribute-like syntax":
int f(int i)
[[ pre: i >= 0]]
[[ post r: r > i ]]
{
[[assert: x > 0]]
return i + x;
}
int f(int i)
pre (i >= 0)
post (r: r > i)
{
assertexpr (x > 0) // *see below
return i + x;
}
// *this might end up being `assertexpr`, `contract_assert`, or some other keyword that isn't exactly `assert`, to avoid a name clash with the existing macro from assert.h
