auto

Learn to declare variables with auto where the compiler infers the type.

auto

The auto keyword lets the compiler deduce the variable type from its initial value. When you write auto x = 10;, x becomes int; auto y = 2.5; makes y a double.

This saves you from typing long type names: auto it = vec.begin(); and so on. You let the compiler figure out the type instead of spelling it out.

auto always requires an initial value; auto x; is invalid because the type cannot be deduced. Deduction happens at compile time with no runtime cost.

Limits of auto

auto is powerful but has limits you should know:

An initial value is required: auto x; is invalid
For references use auto&: auto& r = x;
const is not preserved: const int c = 5; auto a = c; here a is int, not const int

For complex types, auto improves readability. But if the type becomes unclear, spelling it out is better. auto is used only for local variables; it cannot be used in function parameters (before C++20).

Exercise Declare variables with auto

Inside main, declare four variables with auto: count with value 10, rate with 2.5, letter with 'Z', ok with true. Print each with std::cout using std::endl.

Show example solution

#include <iostream>

int main() {
    // Declare four variables with auto
    auto count = 10;
    auto rate = 2.5;
    auto letter = 'Z';
    auto ok = true;
    // Print with std::cout
    std::cout << count << std::endl;
    std::cout << rate << std::endl;
    std::cout << letter << std::endl;
    std::cout << ok << std::endl;
    return 0;
}

Test yourself

Match auto deductions

auto x = 10;

int
auto y = 2.5;

double
auto z = 'A';

char
auto ok = true;

bool
What does auto do?
- It gives the memory address of the variable
- It deletes the variable
- It deduces the type from the initial value
- It makes the variable constant
Show answer

It deduces the type from the initial value — auto deduces the type from the initial value.
Declaring with auto
```
____ count = 10;
```
Show answer
```
auto count = 10;
```

Next lesson: const

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