concatenating strings using “+” in c++ [duplicate]

Answer 1

"Hello" and ", world" in the first example are const char* objects. There is no defined way to add two character arrays together due to the nature of pointers, although logically it seems fine.

In the second example, hello is a std::string objects which has a + operator defined so that when you write:

hello + ", world"

it creates a new std::string object containing both pieces of content.

Answer 2

In the first expression

"Hello" + ", world"

the compiler needs to find a suitable function such as operator+(const char *, const char *). No such function exists, so it cannot compile.

Conversely,

hello + ", world"

is looking for one matching operator+(const std::string&, const char*), and that overload does exist (it is provided by std::string).

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Note that you even if you could write your own overload:

std::string operator+ (const char *left, const char *right)
{
    return std::string(left) + right;
}

(and you can't, as Praetorian points out) it wouldn't be a good idea.

Firstly, with primitive arguments, you'd lose ADL (ie, if the standard library put the operator in namespace std, it wouldn't normally be visible outside).

Secondly, if you have other libraries with their own string representations (eg. pre-STL stuff like RogueWave, or Qt, etc. etc.) they'd be equally justified in providing an overload for their string type, and the compiler would have no way to choose between them.

Answer 3

It's because "Hello" is not, when the compiler reads it, a std::string, but const char * - which means you can't use it for +.

You can trivial fix it with:

 const std::string message = std::string("Hello") + ... 

Answer 4

"Hello" is not a string, in the sense that it's not an object of type std::string. It's a string literal, which is an array of characters. You can't concatenate two literals with +, but you can concatenate a std::string with an array (and vice versa).

"Hello" + ", world" + exclam is equivalent to ("Hello" + ", world") + exclam, and so doesn't work because it tries to concatenate two literals. However, you could concatenate them without the + operator: "Hello" ", world" + exclam

hello + ", world" + "!"; is equivalent to (hello + ", world") + "!". It concatenates a std::string with a literal; the result is a new std::string which is then concatenated with the second literal. Both concatenations are allowed.

The reason is that C++ is a member of a language family that has slowly evolved over the last half century or so, and still has vestigial remnants of ancient languages around the edges.