Difference Between Lazy Evaluation And Eager Evaluation In C++

In this article, we will discuss the differences between Lazy Evaluation and Eager Evaluation in C++. Before discussing their differences, we must know about Lazy Evaluation and Eager Evaluation in C++ with their examples.

What is the Lazy Evaluation?

Lazy evaluation evaluates an expression only when its value is actually needed. It allows applications to potentially avoid unnecessary computations and boost efficiency by running until the very last minute.

It is called lazy evaluation when an expression is not evaluated until its value is actually needed. It can help save efforts on unnecessary computations, especially in such cases when all the phrases to be evaluated are not needed. Strictly speaking, C++ does not support lazy evaluation by default, but it is still very much possible through mechanisms like lambdas, std::function, and proxy objects. It plays an important role when working with operations that consume much and require resources or when simplifying conditional logic. When the unwanted value is present, it puts off the execution, and hence allows the statements to be processed in a manner that short-circuits them, which makes with importance cheap on memory.

Key features of Lazy Evaluation:

Several key features of Lazy Evaluation in C++ are as follows:

• ✅ Deferred Execution: Expressions are evaluated only when the program needs the result for it to continue execution.
• ✅ Efficiency: It may save unnecessary computations, especially in cases where short-circuiting evaluation is applicable. When certain elements of an expression are not needed.
• ✅ Memory usage: Sometimes helps reduce memory consumption by not computing redundant values.
• ✅ Example using C++: Although C++ does not allow lazy natively, we can emulate it with functions like lambda expressions, std::function, and specially constructed proxy objects .

Example 1:

Let us take an example to illustrate the Lazy Evaluation in C++.

#include <iostream>
#include <functional>

std::function<int()> lazyEval = []() { 
    std::cout << "Expensive computation\n"; 
    return 42; 
};

int main() {
    std::cout << "Before evaluation\n";
    std::cout << lazyEval() << std::endl;  // Triggers the computation
    return 0;
}

Output:

Before evaluation
Expensive computation
42

Example 2:

Let us take an example to illustrate the Lazy Evaluation in Conditional Logic in C++.

#include <iostream>
bool isEven(int num) {
    std::cout << "Checking if number is even...\n";
    return num % 2 == 0;
}

bool expensiveCheck() {
    std::cout << "Performing an expensive check...\n";
    return true;  // This is an expensive operation
}

int main() {
    int number = 4;
    
    // Lazy evaluation: the expensiveCheck() is not called because isEven() is false
    if (number == 4 && expensiveCheck()) {
        std::cout << "Both conditions are true\n";
    } else {
        std::cout << "Short-circuited, second condition not evaluated\n";
    }

    return 0;
}

Output:

Short-circuited, second condition not evaluated

Explanation:

In this example, Logic short-circuiting is used to illustrate lazy evaluation. It avoids needless processing because the first condition (number == 4) is false and the costlyCheck method is not evaluated.

Example 3:

Let us take an example to illustrate the Fibonacci Series using Lazy Evaluation in C++.

#include <iostream>
#include <functional>

// Fibonacci generator with lazy evaluation
std::function<int()> lazyFibonacci(int a = 0, int b = 1) {
    return [a, b]() mutable {
        int next = a;
        a = b;
        b = next + a;
        return next;
    };
}

int main() {
    auto fib = lazyFibonacci(); // Create a lazy Fibonacci generator
    
    std::cout << "First 5 Fibonacci numbers (Lazy Evaluation):\n";
    for (int i = 0; i < 5; ++i) {
        std::cout << fib() << " ";  // Lazy evaluation: each Fibonacci number is computed on-demand
    }
    std::cout << std::endl;
    
    return 0;
}

Output:

First 5 Fibonacci numbers (Lazy Evaluation):
0 1 1 2 3

What is the Eager Evaluation?

When we encounters an expression, we evaluates it eagerly. It is a standard evaluation procedure for C++ and most other programming languages .

When an expression appears in a C++ program, it is evaluated quickly by default, which means it happens right away. Whether the results are used later, this approach ensures predictability because all expressions are computed beforehand. Eager assessment can lead to meaningless calculations and memory loss, even while it makes program flow logic easier to grasp if some variables are never used. It is a feature of most imperative languages that guarantees side effects occur immediately. It makes debugging easier, but it is less useful when dealing with complex logic or data structures where all values are not needed.

Key Features of Eager Evaluation:

Several key features of Eager Evaluation in C++ are as follows:

• ✅ Instantaneous execution: An expression is evaluated as soon as it enters the program.
• ✅ Predictability: Because every statement is being pre-evaluated, the behavior is definite and predictable.
• ✅ Memory usage: Computing numbers that will never be used could lead to wasteful computations and memory usage.
• ✅ C++ illustration: Because C++ by default utilizes quick evaluation, expressions and arguments provided to functions are evaluated immediately upon writing.

Example 1:

Let us take an example to illustrate the Eager Evaluation in C++.

#include <iostream>
int expensiveComputation() {
    std::cout << "Expensive computation\n";
    return 42;
}

int main() {
    std::cout << "Before evaluation\n";
    int result = expensiveComputation();  // Computation happens immediately
    std::cout << result << std::endl;
    return 0;
}

Output:

Before evaluation
Expensive computation
42

Example 2:

Let us take another example to illustrate the Eager Evaluation of function arguments in C++.

#include <iostream>

int computeValue() {
    std::cout << "Computing value...\n";
    return 42;
}

void useValue(int value) {
    std::cout << "Using value: " << value << std::endl;
}

int main() {
    std::cout << "Before calling useValue\n";
    
    // Eager evaluation: computeValue() is executed immediately
    useValue(computeValue());
    
    return 0;
}

Output:

Before calling useValue
Computing value...
Using value: 42

Example 3:

Let us take an example to illustrate the Fibonacci Series using Eager Evaluation in C++.

#include <iostream>
#include <vector>

// Eager Fibonacci calculation
std::vector<int> eagerFibonacci(int n) {
    std::vector<int> fib(n);
    fib[0] = 0;
    fib[1] = 1;
    for (int i = 2; i < n; ++i) {
        fib[i] = fib[i-1] + fib[i-2];  // Eager evaluation: Fibonacci numbers are computed upfront
    }
    return fib;
}

int main() {
    int count = 5;
    
    std::cout << "First 5 Fibonacci numbers (Eager Evaluation):\n";
    std::vector<int> fib = eagerFibonacci(count);  // Eager evaluation: all Fibonacci numbers computed immediately
    for (int i = 0; i < count; ++i) {
        std::cout << fib[i] << " ";
    }
    std::cout << std::endl;
    
    return 0;
}

Output:

First 5 Fibonacci numbers (Eager Evaluation):
0 1 1 2 3

Key differences between Lazy Evaluation and Eager Evaluation:

There are several key differences between Lazy Evaluation and Eager Evaluation in C++. Some main differences are as follows:

Conclusion:

In conclusion, there are two basic techniques of expression evaluation, which are referred to as eager evaluation and lazy evaluation, and they are both associated with their own advantages and disadvantages. Since lazy evaluation only computes values when they are really needed, it can save time and memory in conditions with too complex requirements, big data structures or infinite sequences. In addition, it lets avoid needless logical expressions. Nonetheless, it may complicate control flow and debugging. On the other hand, Eager evaluation, which is the default of C++, is simple and predictably works by evaluating the expressions immediately they are meet. Eager evaluation consumes less thought effort for program design but may create intermediate states and increase memory even where some results will not be needed.