How to Check if String is Empty in C++

In C++, you can check if a string is empty by using the empty() method, which returns true if the string has no characters. Alternatively, you can check if length() or size() returns zero to determine if the string is empty.

Syntax

To check if a string is empty in C++, use the empty() method or compare the length() or size() to zero.

• str.empty(): Returns true if str has no characters.
• str.length() == 0: Checks if the string length is zero.
• str.size() == 0: Equivalent to length(), checks if string is empty.

cpp

bool isEmpty = str.empty();
// or
bool isEmpty = (str.length() == 0);
// or
bool isEmpty = (str.size() == 0);

Example

This example shows how to check if a string is empty using empty() and length(). It prints messages based on whether the string has content or not.

cpp

#include <iostream>
#include <string>

int main() {
    std::string str1 = "";
    std::string str2 = "Hello";

    if (str1.empty()) {
        std::cout << "str1 is empty." << std::endl;
    } else {
        std::cout << "str1 is not empty." << std::endl;
    }

    if (str2.length() == 0) {
        std::cout << "str2 is empty." << std::endl;
    } else {
        std::cout << "str2 is not empty." << std::endl;
    }

    return 0;
}

Output

str1 is empty.
str2 is not empty.

Common Pitfalls

Some common mistakes when checking if a string is empty include:

• Comparing the string directly to an empty string literal using == "" works but is less efficient than empty().
• Using str == NULL is incorrect because std::string is not a pointer.
• Checking only the first character like str[0] == '\0' can cause errors if the string is empty.

cpp

#include <iostream>
#include <string>

int main() {
    std::string str = "";

    // Wrong: comparing to NULL (compilation error)
    // if (str == NULL) { }

    // Less efficient but works:
    if (str == "") {
        std::cout << "String is empty (using == \"\")." << std::endl;
    }

    // Correct and efficient:
    if (str.empty()) {
        std::cout << "String is empty (using empty())." << std::endl;
    }

    return 0;
}

Output

String is empty (using == "").
String is empty (using empty()).

Quick Reference

Method	Description	Returns
empty()	Checks if string has no characters	true if empty, false otherwise
length() == 0	Checks if string length is zero	true if empty, false otherwise
size() == 0	Same as length(), checks if string is empty	true if empty, false otherwise
== ""	Compares string to empty string literal	true if empty, false otherwise (less efficient)

Key Takeaways

Use empty() method to check if a string is empty efficiently and clearly.

Checking length() or size() equal to zero also works to detect empty strings.

Avoid comparing std::string to NULL or checking characters directly.

Comparing to an empty string literal "" works but is less efficient than empty().

Always use standard string methods for safe and readable code.