CODEWITHSUNDEEP
c++string
Mat
Mat

Reputation: 73

Plus operator on strings in c++

Can you explain me the Exercise 1-2 from accelerated c++? 

int main()
{
    const std::string exclam = "!";
    const std::string message = "Hello" + ", world" + exclam;
    std::cout << message << std::endl;
}

Why is this not correct? Changing the variable with "Hello" works fine.

Is it because of the fact that the operator + is right-associative?

Upvotes: 1

Views: 1208

Answers (2)

geauser
geauser

Reputation: 1025

Adding to the explanation of @songyuanyao, you can do this either by using the + operator of std::string:

const std::string left  = "Hello ";
const std::string right = "World";
const std::string point = "!";
std::cout << left + right + point << std::endl;

or by manipulating the output directly with std::cout:

const std::string left  = "Hello";
const std::string right = "World";
const std::string point = "!";
std::cout << left << " " << right << point << std::endl;

Or by using the printf function (don't forget to #include <stdio.h>):

const std::string left  = "Hello";
const std::string right = "World";
const std::string point = "!";
printf("%s %s%s\n", left.c_str(), right.c_str(), point.c_str());

Look at this post if you want to know what the c_str() is about.

Upvotes: 1

songyuanyao
songyuanyao

Reputation: 172864

The associativity of operator+ is left-to-right. Then "Hello" + ", world" + exclam is interpreted as ("Hello" + ", world") + exclam while "Hello" + ", world" is invalid. "Hello" and ", world" are const char[]s and could decay to pointer as const char* which can't be added.

Using std::string instead of c-style string, or changing the code to "Hello" + (", world" + exclam) works because there's operator+ for std::string which could accept two std::strings or a std::string and a c-style string (as either the 1st or 2nd operand), and it returns std::string which could be added further.

Upvotes: 3

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