Why can't a string literal initializer list for a std::vector create a std::vector<std::string> in C++?

Question

In C++, if I do:

std::vector words {"some","test","cases","here"};

Can anyone explain why words is not a std::vector<std::string> type of container?
Isn't C++ supposed to deduct the type through the initializer-list I gave?
If "some", "test" are not string literals, what do std::string literals look like?

what type should it deduce? You are aware that some of the literals are of different type? — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Oct 30, 2019 at 13:08
I think the issue is clearer with integer values. The compiler would not know which integer type to use. — S. K.
– S. K., Commented Oct 30, 2019 at 13:15
@formerlyknownas_463035818 Well, aren't "some","test,"cases" are of std::string type? I write std::string x = "Hello" and it works without any issue. What am I missing here? — MrProgrammer
– MrProgrammer, Commented Oct 30, 2019 at 13:15
@UtkarshGupta std::string myStr = "sampletext" you declare type, can't be ambiguous or something else. I suppose it passes a C-string to std::string constructor and then builds an std::string - but I am not sure about this. — Koenigsberg
– Koenigsberg, Commented Oct 30, 2019 at 13:31

NathanOliver · Accepted Answer · 2019-10-30 13:19:27Z

11

a string literal, like "something", is a c-string. It creates a const char[N] with static storage duration where N is the number of characters plus a null terminator. That means when you do

std::vector words {"some","test","cases","here"};

What you've done is create a std::vector<const char*> since arrays can decay to pointers.

If you want a std::vector<std::string> then what you need is to use the std::string user defined literal. That would look like

using namespace std::string_literals;
std::vector words {"some"s,"test"s,"cases"s,"here"s};

and now you have actually std::strings that the compiler will use to deduce the type of the vector.

answered Oct 30, 2019 at 13:19

NathanOliver

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Bacon Over a year ago

Note that std::string user defined literals are only available from c++ 14 (which is fine here given OP's C++17 tag, but good to note for future readers)

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