Скачать или смотреть Why does concatenation of a string and char in C+ + return an empty string?

Discover why concatenating a `string` and `char` in C+ + leads to unexpected results and learn the proper way to append characters to strings.
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Understanding Concatenation of string and char in C+ +

C+ + is a powerful programming language, but it can also be quite tricky when it comes to working with strings. A common question that developers often encounter is: Why does concatenation of a string and a char in C+ + return an empty string? This guide aims to clarify this issue and provide you with the correct approach to concatenate a char to a string in C+ + .

The Problem

Consider the following piece of code:

[[See Video to Reveal this Text or Code Snippet]]

At first glance, you might expect that the variable s would store the value "hello4", but that's not the case. Instead, it results in an empty string (""). Let's dive deeper into what's happening here.

What's Happening Under the Hood

String Representation:

The string literal "hello" is represented as an array of characters: {'h', 'e', 'l', 'l', 'o', '\0'} (with '\0' being the terminating null character).

Character Value:

The character literal '4' has a numeric ASCII value of 52.

Pointer Arithmetic:

When you write "hello" + '4', the compiler interprets it as "hello" + (char)52.

In C+ + , adding a char to a string literal results in pointer arithmetic. Therefore, you are effectively moving 52 positions away from the start of the string literal, which exceeds its length of 6 characters (including the null terminator).

Undefined Behavior:

This calculation gives you a pointer that points to a non-existent character. When you initialize the std::string s with this pointer, it is assumed (incorrectly) to be a pointer to a valid null-terminated string. This leads to undefined behavior, where the result can be unpredictable.

The Reality of Undefined Behavior

The assertion assert(s == ""); is not guaranteed to be true. In this case, it coincidentally returns true because of how the memory situation was handled by the compiler and your environment. However, it’s crucial to understand that it could produce any result, including crashing your program or performing unexpected actions.

The Correct Approach

To successfully concatenate a char to a string, you need to ensure that the operation is performed on std::string objects. Here are two correct methods to achieve this:

Method 1: Using the std::string Constructor

[[See Video to Reveal this Text or Code Snippet]]

Here, we explicitly create a std::string object from the string literal "hello".

The std::string class has an overloaded operator+ that can concatenate a char, returning a new std::string.

Method 2: Using String Literals with Suffix (C+ + 14 and above)

If you're using C+ + 14 or later, you can also use:

[[See Video to Reveal this Text or Code Snippet]]

In this case, the s suffix creates a std::string type directly from the literal "hello".

This method also supports direct concatenation with a char.

Conclusion

Understanding how C+ + handles string and char concatenation is vital for avoiding pitfalls and ensuring your code behaves as expected. Remember:

Directly adding a char to a string literal results in pointer arithmetic and undefined behavior.

Always use std::string for concatenation to maintain clarity and correctness in your code.

By following these guidelines, you'll be able to work confidently with string and character manipulation in C+ + . Happy coding!