Automatic Type Deduction

Although C++ is a statically typed language, you can use auto to let the compiler deduce the type of a variable during initialization. This feature was introduced in C++11 and is not available in older versions.

main


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#include <iostream>

int main()
{
    auto num = 9;
    auto str = "I am string";
    
    std::cout << num << std::endl;
    std::cout << str << std::endl;
}

While this can save time and make the code more concise, it can also obscure the intended type, making errors harder to spot.

main


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#include <iostream>

int main()
{
    auto num1 = 9;
    auto num2 = 12;
    auto num3 = 15000;
    auto calculations = num1 / num2 * num3;
    
    std::cout << calculations;
}

Specifying types would help us understand code better and find what's wrong:

main


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#include <iostream>

int main()
{
    float num1 = 9;
    float num2 = 12;
    float num3 = 15000;
    float calculations = num1 / num2 * num3;
    
    std::cout << calculations;
}

Note

Any of num1, num2 or num3 should be float to output the correct result.

Nevertheless, auto is widely used in range-based for loops since it is a versatile method of iterating over arrays and other containers. Just use the following syntax for iterating over arr:

main

auto


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#include <iostream>

int main()
{
    int num_arr[5] = {1, 5, 10, 15, 20};
    
    for (auto num : num_arr)
        std::cout << num << " ";
}

It also can be used when the type is long to type but is clear from the context and can't lead to incorrect results.

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