Why those two std::string find() functions are declared as noexcept?

Question

According to [string.find#1.2] and [string.find#1.3]:

Each member function of the form

constexpr size_type F(const charT* s, size_type pos) const;

has effects equivalent to: return F(basic_string_view<charT, traits>(s), pos);

Each member function of the form

constexpr size_type F(const charT* s, size_type pos, size_type n) const;

has effects equivalent to: return F(basic_string_view<charT, traits>(s, n), pos);

Those two find functions are not declared as noexcept, but all of three compiler vendors didn't follow the rule, they just implement it as noexcept.

What is the reason behind this?

And adding noexcept wouldn't really violate the standard, as it's still source-code-compatible. — Joseph Larson, Mar 12 '21 at 16:37

Barry · Accepted Answer · 2021-03-12T16:49:44.497

This is the so-called Lakos rule. See N3248. The rule is that if a function has a precondition, it should not be marked noexcept because it prevents an implementation from checking the precondition and throwing an exception on validation, if the implementation would choose to do that.

In this case, find(s, pos, n) is equivalent to find(string_view(s, n), pos). And that string_view(s, n) construction has a precondition that s is a valid string of length at least n. So an implementation could try to validate that somehow and throw an exception on failure.

But none of the implementations actually try to validate this, so they don't take the freedom of potentially throwing on precondition violation, and instead simply mark these functions as noexcept. Which they're allowed to do from [res.on.exception.handling]:

An implementation may strengthen the exception specification for a non-virtual function by adding a non-throwing exception specification.

Library Evolution has recently accepted P1656 which basically reverts the Lakos Rule.

score 0 · Answer 2 · answered Mar 12 '21 at 16:42

Ok, I find the answer.

According to the [res.on.exception.handling]:

An implementation may strengthen the exception specification for a non-virtual function by adding a non-throwing exception specification.

So they are free to mark this overload noexcept. Thank you for your attention.

Why those two std::string find() functions are declared as noexcept?

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