---
title: 1.21 decltype
description: "The `decltype` specifier allows the compiler to determine the data type of an expression without evaluating it. It is useful when the exact type of an expression is unknown, difficult to write, or depends on another variable. Unlike `auto`, which…"
url: "https://edcret.in/docs/cpp-programming/basic-refreshers-cplusplus/1-21-decltype/"
updated: "2026-07-28"
category: C++ Programming / Basic Refreshers
---

# 1.21 decltype

The `decltype` specifier allows the compiler to determine the data type of an expression without evaluating it. It is useful when the exact type of an expression is unknown, difficult to write, or depends on another variable.

Unlike `auto`, which deduces the type from an initializer, `decltype` determines the type from an existing expression. This makes it valuable when declaring variables, function return types, and template-based code.

## Source Code

```
#include <iostream>

int main()
{
    int value = 100;
    decltype(value) copy = 200;

    std::cout << "Value: " << value << '\n';
    std::cout << "Copy: " << copy << '\n';

    return 0;
}
```

## Output

```
Value: 100
Copy: 200
```

## Explanation

- `decltype(value)` determines the data type of the expression `value`.
- Since `value` is of type `int`, `copy` is also declared as an `int`.
- No value from `value` is copied during type deduction; only its type is used.
- `decltype` examines the expression at compile time without evaluating it.
- `decltype` is commonly used when the exact type of an expression is not known in advance.
- `std::cout` displays the values stored in both variables.
- `return 0;` terminates the program successfully.