decltype

decltype deduces the type of an expression, preserving references and const qualifiers exactly.

Difference from auto

auto uses template deduction (drops ref/const), decltype preserves exact type including references.

Basic Usage

int x = 42;
decltype(x) y = x;  // int (same type as x)

const int& ref = x;
decltype(ref) r = x;  // const int& (preserves reference and const)

auto a = ref;         // int (drops reference and const)
decltype(ref) b = ref;  // const int& (keeps everything)

---

decltype Rules

Named Variables

int x = 42;
const int cx = x;
int& rx = x;

decltype(x)   // int
decltype(cx)  // const int
decltype(rx)  // int&

Rule 1: For named variables, decltype gives the declared type exactly.

Expressions

int x = 42;

decltype((x))    // int& (parentheses make it expression)
decltype(x + 0)  // int (expression result)
decltype(x++)    // int (post-increment returns value)
decltype(++x)    // int& (pre-increment returns reference)

Rule 2: For expressions (including parentheses around names), decltype gives value category:

• lvalue expression → T&
• xvalue expression → T&&
• prvalue expression → T

---

Practical Uses

Variable Type Matching

std::vector<int> vec;

// Declare same type as existing variable
decltype(vec) vec2;  // std::vector<int>
decltype(vec.size()) size;  // size_t

// Useful when type is complex
std::map<std::string, std::vector<int>> map;
decltype(map) map2;  // Exact same type

Return Type Deduction

template<typename Container>
auto getElement(Container& c, int index) -> decltype(c[index]) {
    return c[index];
}

// c[index] might return reference
std::vector<int> vec = {1, 2, 3};
auto& elem = getElement(vec, 0);  // Returns int&

Perfect Return Type

template<typename T, typename U>
auto add(T a, U b) -> decltype(a + b) {
    return a + b;
}

auto result = add(5, 3.14);  // double (decltype(int + double))

---

decltype vs auto

const int x = 42;
const int& ref = x;

// auto drops const and reference
auto a = ref;       // int

// decltype preserves everything
decltype(ref) b = ref;  // const int&

// auto with modifiers
auto& c = ref;      // const int&
const auto d = ref; // const int

// decltype for exact type
decltype(ref) e = ref;  // const int& (exact match)

---

Common Patterns

Type of Member

struct Point {
    double x, y;
};

Point p;
decltype(p.x) value;  // double

decltype(Point::x) coord;  // double (static member access)

Container Element Type

std::vector<int> vec;
decltype(vec[0]) elem;  // int& (vector::operator[] returns reference)

std::vector<bool> bvec;
decltype(bvec[0]) proxy;  // std::vector<bool>::reference (proxy)

Function Return Type

int func();

decltype(func()) x;  // int (return type of func)
decltype(func) f;    // int() (function type)
decltype(&func) fp;  // int(*)() (function pointer type)

---

Parentheses Matter!

int x = 42;

decltype(x)   // int (name)
decltype((x)) // int& (expression - lvalue)

// Why? (x) is an lvalue expression referring to x
// decltype of lvalue expression → reference

Critical Difference

decltype(name) ≠ decltype((name))
Parentheses change name to expression!

---

Return Type Deduction (C++11 vs C++14)

C++11: Trailing Return Type

template<typename T, typename U>
auto add(T a, U b) -> decltype(a + b) {
    return a + b;
}

Must use trailing return type because a and b not in scope before parameter list.

C++14: auto Return

template<typename T, typename U>
auto add(T a, U b) {
    return a + b;  // auto deduction (uses template rules)
}

// Returns int for add(5, 10), drops reference
std::vector<int> vec;
auto getFirst() {
    return vec[0];  // Returns int (copy), not int&
}

---

Combining auto and decltype

std::vector<int> vec = {1, 2, 3};

// auto drops reference
auto x = vec[0];  // int (copy)

// decltype preserves reference
decltype(vec[0]) y = vec[0];  // int&

// decltype(auto) - best of both worlds (C++14)
decltype(auto) z = vec[0];  // int& (preserves return type)

---

Use Cases

Generic Code

template<typename Container>
void process(Container& c) {
    // Get exact type of element
    decltype(c[0]) elem = c[0];
    
    // Modify if it's a reference
    elem = 42;  // Modifies container if elem is reference
}

Type Traits

template<typename T>
struct remove_reference {
    using type = T;
};

template<typename T>
struct remove_reference<T&> {
    using type = T;
};

int x;
using Type = typename remove_reference<decltype(x)>::type;  // int

---

Summary

decltype:

• Preserves exact type including references and const
• decltype(name) → declared type
• decltype(expression) → value category type
• decltype((name)) → reference (expression)

Comparison:

const int& ref = x;

auto a = ref;           // int (template deduction)
decltype(ref) b = ref;  // const int& (exact type)
decltype(auto) c = ref; // const int& (C++14)

Use when:

• Need exact type preservation
• Template return type forwarding
• Type of complex expressions
• Generic code requiring perfect forwarding

// Common idioms
decltype(expr) var;           // Match expression type
decltype(auto) var = expr;    // Deduce with preservation (C++14)
auto func() -> decltype(expr) // Trailing return type