From cppreference.com
staticconstexprsize_typenpos=size_type(-1);(since C++17)This is a special value equal to the maximum value representable by the type
. The exact meaning depends on context, but it is generally used either as end of view indicator by the functions that expect a view index or as the error indicator by the functions that return a view index.
Example
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#include<string_view>constexprboolcontains(std::string_viewconstwhat,std::string_viewconstwhere)noexcept{returnstd::string_view::npos!=where.find(what);}intmain(){usingnamespacestd::literals;static_assert(contains("water","in a bottle of water"));static_assert(!contains("wine","in a bottle of champagne"));static_assert(""sv.npos=="haystack"sv.find("needle"));}See also
constexprsize_typenpos[static]the special value size_type(-1), its exact meaning depends on the context