Return the position of the first match.

constexpr size_type
find_first_of(
    string_view str,
    size_type pos = 0) const;
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constexpr size_type
find_first_of(
    char c,
    size_type pos = 0) const;
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constexpr size_type
find_first_of(
    char const* s,
    size_type pos,
    size_type n) const;
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constexpr size_type
find_first_of(
    char const* s,
    size_type pos = 0) const;
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