如何解释这个关于 std::errc 的条件语句

Question

I was reading an example of how to use std::from_chars function here , when I came by the following if statment:

if(auto [p, ec] = std::from_chars(str.data(), str.data()+str.size(), result);
       ec == std::errc())
        std::cout << result << "\n" "p -> \"" << p << "\"\n";

Unfortunately I can not understand what ec == std::errc() means and how is it the condition. Does it mean that if ec (the possible error) is equal to default value of enum class errc the the following line should be executed? if so isn't that also an error?

Can someone please explain it to me?

Answer 1

This

if(auto [p, ec] = std::from_chars(str.data(), str.data()+str.size(), result);
       ec == std::errc())
        std::cout << result << "\n" "p -> \"" << p << "\"\n";

is similar to

auto [p, ec] = std::from_chars(str.data(), str.data()+str.size(), result);
if( ec == std::errc() )
    std::cout << result << "\n" "p -> \"" << p << "\"\n";

---

Long answer.. based on manual reference the result for std::from_chars is defined as

struct from_chars_result {
    const char* ptr;
    std::errc ec;
};

and based on errc manual std::errc() is a condition for success OTOH the individual errors can be compared using ec.code()