The Silent Undefined Behavior Plaguing C++ Code

For decades, fundamental C++ patterns like allocating structs with malloc and accessing members, or reinterpreting network buffers as objects, technically invoked undefined behavior (UB). Consider this ubiquitous C-style code:

struct X { int a, b; };
X* make_x() {
  X* p = (X*)malloc(sizeof(X));
  p->a = 1;  // UB pre-P0593: No X or int object exists!
  p->b = 2;
  return p;
}

Similarly, std::vector implementations and deserialization routines relied on pointer arithmetic over "raw" storage—another UB minefield. The root issue? C++'s object model strictly required explicit object creation via definitions, new, or unions.

Enter Implicit-Lifetime Types and Operations

P0593R6 solves this by introducing implicit-lifetime types:
- Scalars, aggregates (arrays/structs), or classes with trivial constructors/destructors
- Essentially types where creation/destruction requires no code execution

Key operations now implicitly create objects in their storage regions when necessary to avoid UB:

// All these now implicitly create objects:
X* p1 = (X*)malloc(sizeof(X));       // Allocation functions
char* buf = new char[sizeof(Foo)];   // Byte-like arrays
memcpy(dest, src, size);             // Memory operations
auto x = std::bit_cast<Foo>(bytes);  // Type-punning utility

The compiler analyzes how storage is used and "materializes" objects retroactively if needed to define behavior. Crucially:
- Only applies where no explicit object exists
- Preserves type-based aliasing (no free type-punning)
- Array new char[N] creates objects within the buffer

Real-World Impact: From Vectors to Deserialization

Validating std::vector Internals

Pre-P0593, std::vector::reserve() performed UB by calculating pointers over storage lacking array objects. Now, allocator::allocate(n) implicitly creates a T[n] object:

// Inside std::vector::reserve():
char* newbuf = allocator.allocate(new_cap); 
// Now implicitly creates a T[new_cap] in newbuf

Safe Network/File Deserialization

Reading bytes from a stream and type-punning via reinterpret_cast is now defined:

void process(Stream* stream) {
  auto buffer = stream->read(); // Returns byte[]
  if (buffer[0] == FOO)
    process_foo(reinterpret_cast<Foo*>(buffer.get())); // Legal
}

Explicit Control: std::start_lifetime_as

For advanced scenarios like in-place type changes, the paper proposes:

template<typename T>
T* start_lifetime_as(void* p);

This explicitly starts T's lifetime at p while preserving underlying bytes—ideal for custom allocators and storage reuse.

The Road to Standardization

  • Core language changes (object model, implicit-lifetime types) target C++20 as a Defect Report
  • Library additions (std::start_lifetime_as) deferred to C++23
  • Major compilers expected to adopt semantics once ratified

"This resolves a tension between formal correctness and practical necessity that persisted for over two decades." — P0593R6

This fix retroactively validates decades of existing code while strengthening C++'s memory model. For systems programmers and library authors, it eliminates a class of UB that was previously unavoidable in performance-critical paths.

Source: P0593R6: Implicit creation of objects for low-level object manipulation