uninitMemberVarNoCtor flag members that cannot take a default member initializer

[Long Huang]

uninitMemberVarNoCtor asks for a default member initializer on members of a constructor-less aggregate. Two kinds of member cannot take one:

1. Bit-fields under --std=c++14/c++17: default member initializers
   for bit-fields only became valid in C++20. Cppcheck warns even with
   --std=c++14, so the suggested fix does not compile.
2. Flexible array members (char Data[];, GNU/C99 extension common in
   wire/IPC headers): they have no size, so an initializer is impossible in
   any standard.

A user-written constructor is not a practical alternative for (1) because it removes aggregate initialization, which these wire-layout structs rely on.

Reproduction 1: bit-fields with --std=c++14

bitfield.cpp:

#include <cstdint>
struct Wire
{
   std::uint8_t Cmd = 1;
   std::uint8_t Mode : 4;
   std::uint8_t Reserved : 4;
};
Wire Make() { return Wire{2, 3, 0xF}; }
int main() { return Make().Mode; }

cppcheck --enable=style --std=c++14 --template=gcc --quiet bitfield.cpp

Actual:

bitfield.cpp:5:17: warning: Member variable 'Wire::Mode' has no initializer. [uninitMemberVarNoCtor]
bitfield.cpp:6:17: warning: Member variable 'Wire::Reserved' has no initializer. [uninitMemberVarNoCtor]

The suggested fix is rejected by the compiler in C++14:

struct W { unsigned char M : 4 = 0; };

$ g++ -std=c++14 -pedantic-errors -fsyntax-only bfinit.cpp
bfinit.cpp:1:34: error: lvalue required as left operand of assignment

Expected:

No uninitMemberVarNoCtor for bit-fields when --std is older than
C++20 (or at least a message that does not recommend an ill-formed fix).

Reproduction 2: flexible array member

flex.cpp:

#include <cstdlib>
#include <cstring>
struct Payload
{
   int Size = 0;
   char Data[];
};
int main()
{
   Payload* p = static_cast<Payload*>(std::malloc(sizeof(Payload) + 4));
   if(p == nullptr)
   {
      return 1;
   }
   p->Size = 4;
   std::memcpy(p->Data, "abc", 4);
   const int r = p->Data[0] == 'a' ? 0 : 1;
   std::free(p);
   return r;
}

cppcheck --enable=style --std=c++14 --template=gcc --quiet flex.cpp
g++ -std=gnu++14 -Wall -Wextra -o flex flex.cpp && ./flex   # exit 0

Actual:

flex.cpp:6:9: warning: Member variable 'Payload::Data' has no initializer. [uninitMemberVarNoCtor]

Expected:

No warning, a flexible array member has no storage in the struct itself and cannot have an initializer.

Version

Cppcheck 2.22.0 (Linux)

[CHR]

Thanks for reporting, ticket is here: https://trac.cppcheck.net/ticket/15177