std::copy_assignable_v not working for reference types, how to fix?

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Summary

Experienced C++ developers often rely on std::is_copy_assignable_v to verify if a type supports copy assignment. However, this trait unexpectedly returns true for reference types (e.g., int&), leading to incorrect assumptions about object reassignability. This occurs because the trait validates assignment expression legality, not type reseatability. This article dissects the problem, demonstrates a safer approach, and explains why this edge case trips up beginners.

Root Cause

The core issue lies in how std::is_copy_assignable_v<T> is computed:

  • It checks whether declval<T&>() = declval<const T&>() is valid.
  • For int&, the expression int_ref = another_int modifies the referenced int, not the reference itself.
  • References themselves cannot be reseated (1), but the assignment expression compiles because it modifies the referred object.
  • In code: 
    int a = 10, b = 20;  
int& ref = a;  
ref = b; // Compiles! But assigns 20 to a, doesn’t make ref point to b  
// ref still points to a, now with value 20

    Thus, std::is_copy_assignable_v<int&> returns true despite references being non-reseatable.

Why This Happens in Real Systems

  1. Trait semantics: The standard defines is_copy_assignable purely for expression validation, not semantic intent.
  2. Template generality: Traits must support both references and non-references uniformly. Disentangling reseating from assignment would fragment the trait.
  3. Historical design: Pre-C++11 move semantics blurred the distinction for reference-like proxy objects.

Real-World Impact

Ignoring this behavior causes:

  • Misleading APIs: Generic code accepting is_copy_assignable_v<T> may assume T supports reassignment, breaking when T is a reference.
  • Silent bugs: Containers/allocators using std::is_copy_assignable_v for reference types compile but corrupt data: 
    template<typename T>  
void unsafeAssign(T& dst, const T& src) {  
  if constexpr (std::is_copy_assignable_v<T>)  
    dst = src; // If T=int&, compiles but doesn’t reseat dst  
}
  • Broken invariants: Reassignment-by-proxy (like IntProxy) is incorrectly blocked (std::is_copy_assignable_v<IntProxy> = false), while raw references pass.

Example or Code

Problem Demonstration: 

#include <print>  
#include <type_traits>  

struct IntProxy {  
    int& i; // Proxy has a reference member  
};  

int main() {  
    // Correct: Assignment operator deleted -> false  
    std::println("{}", std::is_copy_assignable_v<IntProxy>); // false  

    // Problem: Returns true despite reference non-reseatability  
    std::println("{}", std::is_copy_assignable_v<int&>); // true?!  
}

Robust Fix: Combine traits to exclude references: 

template <typename T>  
constexpr bool is_copy_assignable_val_v =  
    std::is_copy_assignable_v<T> && !std::is_reference_v<T>;  

// Usage  
static_assert(!is_copy_assignable_val_v<int&>);  // Now passes  
static_assert(is_copy_assignable_val_v<int>);     // Still works

Full example compiler-testable here.

How Senior Engineers Fix It

  1. Compose traits: As above, combine is_copy_assignable with !is_reference.

  2. Concept composition (C++20): 

     template<typename T>  
 concept CopyAssignableVal = std::is_copy_assignable_v<T> && !std::is_reference_v<T>;  

 template<CopyAssignableVal T>  
 void safeReassignment(T& target, const T& source) {  
     target = source; // Enforced to be non-reference  
 }

  3. Static_assert customization points: 

     class CustomType {  
 public:  
     CustomType& operator=(const CustomType&) { /* ... */ }  
 };  

 template<typename Container>  
 void containerSetup() {  
     static_assert(  
         is_copy_assignable_val_v<typename Container::value_type>,  
         "Elements must be copy-assignable (and non-reference)"  
     );  
 }

    Key principle: Never interpret is_copy_assignable in isolation for reseating checks.

Why Juniors Miss It

  1. Literal interpretation: Taking is_copy_assignable_v at face value (“can I assign to this?”) without considering reference edge cases.
  2. Expression vs. semantics confusion: Overlooking that = syntax for references modifies referees not references.
  3. Undocumented assumptions: Not realizing that types like IntProxy are explicitly deleted, while raw references flood trait results.
  4. Testing gaps: Only validating scenarios with value types (int, string), missing references/proxies.

Senior takeaway: Reference handling in C++ traits is syntactic, not semantic. Always reinforce traits with domain-specific constraints.

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