For historical reasons, and for compatibility with C, string literals are not standard library strings in C++.

This passage shows a simple way to accomplish this.

A method to add string literals in C++

Preface

First of all, here is my first tech passage in fully English.

So, i chose a very small topic as a new start, hope that you can follow my step to switch your language setting to a new foreign language.

Now, enjoy!

Try add string literals by +

A demo to reveal the question

Considering code below:

string s = "hello" + ", world!";

The compiler rejected the code, add threw a error:

Invalid operands to binary expression ('const char[6]' and 'const char[9]')

This is counter-intuitive!

Why we can’t add two string literals, since the STL has already overrided + operator for string?!

The reason why we can’t add

Well, this is a historical problem: C++ wants to be compatible with char array in C!

According to C++ Primer, we know that:

• All the string literals’ type in C++ is const char[];

And the code string s="hello"; actually equals to the code below:

string s("hello");

In another word, we are use basic_string(const _CharT* __s) to construct a new string!

So, the code "hello" + ", world!" is invalid, because literal string is const char[] type and we can’t add two const char[] type!

"hello" is the type of const char[6]?

If you are a C developer, the answer is obvious:

The C uses char array as the string.

So, to indicate this is a string, we added a special character \0 at the end of the char array.

This is why "hello" is typed const char[6];

How to add literal string using +?

Thanks to the operator override in C++, we can simply override the ""s operator.

Yes, not "", but ""s!

And the STL has already done this job for us!

So all we need to do is to use it as using std::operator""s;, and change our code as :

string s = "hello"s + ", world!";

Then we can compile the code, and use s as a string!

Noticed that your compiler need to support C++11 at least!

Summary

At last, here is a.summary.

Above the passage, we learnt:

• All the string literals’ type in C++ is const char[];
• The C uses char array as the string, and we added a special character \0 at the end of the char array to mark it;
• We can use the std::operator""s to add string literals in C++;

Appendix

Reference:

• https://www.v2ex.com/t/850206
• C++ Primer