How To Use C++ Booleans: The Experts' In-depth Guide

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In this article, we will explore the anatomy of C++ booleans, starting from a simple declaration of c++ boolean variable and eventually implementing operator overloading functions using boolean types.

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Booleans are common among programming languages, as they provide a distinct data type for storing true or false values. C++ language also provides a boolean built-in type using a keyword – bool and it can be a very powerful tool when used correctly. Keep reading to see how our programming experts advise you to apply them!

JUMP TO TOPIC

• How to Declare Boolean in C++?
  • Code:
• Use C++ booleans as return values for functions
  • Code:
  • Program Output:
• Use C++ booleans to implement comparison operators
  • Code:
  • Program Output:
• Include C++ booleans in bit-fields to access single bits
  • Code:
  • Program Output:
• Implicit conversion rules for C++ booleans
  • Code:
  • Program Output:
• Conclusion

How to Declare Boolean in C++?

Like any other variable, we can declare C++ boolean type using a bool keyword followed by the name of the variable itself. C++ boolean variables can be initialized by assigning integer values to them or by special keywords called boolean literals. The latter ones are represented in C++ with keywords true/false, and their type is bool.

We should note that assigning true or 1 to the boolean variable will have the same effect, but it’s preferable to use the true keyword when initializing variables.

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On the other hand, assigning a zero value or null pointer to the boolean variable will set its value to false. The said feature of C++ booleans is demonstrated in the following code snippet, which also prints the values of the declared boolean variables to the console.

Mind that, var1 is not initialized and since the C++ standard does not specify, you should not expect it to have true or false value when printed to the cout stream.

Let’s look at some variable declarations more closely:

bool var1; // declaration of a boolean variable> bool var2 = true; // declaration and initialization of a boolean variable bool var3 = 1; // alternative declaration and initialization of a boolean variable> cout << “var1 – ” << var1 << endl; cout << “var2 – ” << var2 << endl; cout << “var3 – ” << var3 << endl;

Alternatively, we can initialize C++ booleans using the values of certain expressions. For example, expressions that utilize comparison operators can be used to assign the resulting c++ boolean values to the given bool variables. Additionally, we can also flip the given bool value by prefixing the variable with the “!” operator as shown in the next sample code:

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• Code:

int x = 21; int y = 32; bool var1 = 2 < 4; bool var2 = x == y; bool var3 = !(var2); cout << “var1 – ” << var1 << endl; cout << “var2 – ” << var2 << endl; cout << “var3 – ” << var3 << endl;

If you compile the previous example code and run it, the displayed c++ boolean values will be numeric. This default behavior can be altered using the input/output stream manipulators like std::noboolapha and std::boolapha. You can use these manipulators by including them after the stream insertion operator and the following boolean variables will be printed correspondingly. Look at this code, for example:

bool var2 = true; bool var3 = 1; cout << “var2 – ” << std::noboolalpha << var2 << endl; cout << “var3 – ” << std::boolalpha << var3 << endl;

Use C++ booleans as return values for functions

C++ boolean functions that need to return only logical true or false values are best suited to be defined using C++ booleans. These functions are mostly used to check for some condition and retrieve the corresponding status with a binary logical value. One such example is a contains() member function for std::set container of the C++ Standard Template Library.

Alternatively, we can define our own functions that return c++ boolean values by specifying the bool keyword before the function name. In the following snippet, we define a function vectorIncludes() to search a key in the vector container and return the result as a binary logical value – true or false.

Notice that the given function implementation utilizes the std::find algorithm to search the vector, but you can construct the function with the desired algorithm or subroutine as long as you return boolean values. Let’s look at the code and its output below that showcases what we just explained:

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• Code:

#include <iostream> #include <algorithm> using std::cout; using std::endl; using std::vector; template<typename T> bool vectorIncludes(vector<T> &v, const T& key) { if (std::find(v.begin(), v.end(), key) != v.end()){ return true; } else { return false; } } int main() { int key = 72; vector<int> arr1 = {10, 22, 77, 123, 15, 62}; vectorIncludes(arr1, key) ? cout << “arr1 includes ” << key << endl : cout << “arr1 does not include ” << key << endl; return 0; }

• Program Output:

arr1 does not include 72

Use C++ booleans to implement comparison operators

C++ booleans are also useful for implementing overloaded operators for the custom classes. Mostly, you will need to utilize bool as a return type for the comparison operators like equal to (==) operator, for example, as shown in the next coding example.

Note that, we defined a class named Rectangle to implement an overloaded comparison operator. Rectangle class defines one constructor function and one member function for overloading – operator==(). This operator checks if the area data member is equal for the given Rectangle objects and returns a bool value as a result. Also, notice in the code that will follow, that the friend keyword is only used to allow the overloaded operator function to access the private members of Rectangle objects.

• Code:

#include <iostream> using std::cout; using std::endl; class Rectangle { public: Rectangle(double x, double y) : w{x}, l{y}, area{x*y} {} friend bool operator==(const Rectangle &rec1, const Rectangle &rec2) { return rec1.area == rec2.area; } private: double w; double l; double area; }; int main() { Rectangle r1(2.4, 3.2); Rectangle r2(3.2, 2.4); Rectangle r3(3.4, 1.2); r1 == r2 ? cout << “Rectangle areas equal” << endl : cout << “Rectangle areas not equal” << endl; r1 == r3 ? cout << “Rectangle areas equal” << endl : cout << “Rectangle areas not equal” << endl; return 0; }

• Program Output:

Rectangle areas equal Rectangle areas not equal

Include C++ booleans in bit-fields to access single bits

Bit-fields are a common way for C++ programmers to access memory units that are smaller than one byte. These handy data structures are built using the struct keyword and they can bundle multiple variables as fields. As a result, we can have multiple single-bit bool variables packed together in a single structure.

Then we can combine the given struct with a union feature to store another uint8_t data field, which will be utilized to access the whole 8-bit value. Essentially, we implement a new type called BitSet, that can be used for storing and manipulating bitmasks. Namely, you can retrieve each bit of uint8_t object by accessing the bool fields starting from b1 to b2, and even flip their values using the simple assignment operation.

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We should let you know, though, that bit-fields can be tricky to work with, as some of the characteristics are implementation-defined and not specified by the language standard. E.g. Bit-field object memory representation may differ on platforms. Also, you should have in mind that bit-field member access is a more computer-heavy process than accessing a member of a struct.

• Code:

#include <iostream> using std::cout; using std::endl; union BitSet{ struct { bool b1:1; bool b2:1; bool b3:1; bool b4:1; bool b5:1; bool b6:1; bool b7:1; bool b8:1; }; uint8_t byte; }; string toBinary(int n) { string r; while ( n!=0 ){ r += ( n % 2 == 0 ? “0” : “1” ); n /= 2; } return r; } int main() { BitSet bs1{}; bs1.byte = 0b11111111; cout << toBinary(bs1.byte) << endl; bs1.b1 = !bs1.b1; bs1.b2 = !bs1.b2; cout << toBinary(bs1.byte) << endl; return 0; }

• Program Output:

11111111 00111111

Implicit conversion rules for C++ booleans

Generally, as a programmer, you can mix integral, floating-point, and boolean types in expressions and assignments. Consequently, these cases need some predefined rules for handling implicit conversions. The latter operations can be value-preserving and narrowing, which can not convert the value to its original type without losing some bits.

The value-preserving ones are often called promotions. So, if we add a bool type to the integral type, the former is converted to an int, boolean literals true/false associated to 1 and 0 values, respectively. Similarly, we can have different integral values stored in a bool variable.

In the following code snippet, we demonstrate several such scenarios and print the corresponding boolean results to the cout stream.

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Notice that std::boolalpha manipulator is only used here to clearly display conversion rules.

• Code:

#include <iostream> using std::cout; using std::endl; int main() { int x = 21; int y = 0; bool var1 = 22; bool var2 = y; bool var3 = x; bool var4 = y + var3; x = x + var4; cout << “var1 – ” << std::boolalpha << var1 << endl; cout << “var2 – ” << std::boolalpha << var2 << endl; cout << “var3 – ” << std::boolalpha << var3 << endl; cout << “var4 – ” << std::boolalpha << var4 << endl; cout << “x – ” << x << endl; return 0; }

• Program Output:

var1 – true var2 – false var3 – true var4 – true

Conclusion

So far we’ve covered core features and common use cases of C++ booleans, through the examples we presented. Here are some useful specifics you might want to remember about C++ booleans:

• C++ booleans can be declared using a keyword – bool
• bool data type is promoted to the int when used in conjunction with integral types
• Boolean literals true and false shall be used to initialize C++ booleans

You should always add new tools to your programming toolkit, and this guide of C++ booleans represents a cohesive explanation of the central part of the language itself. Keep working on your C++ skills by integrating the boolean types that we mentioned, and happy coding to you!

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