Why do designated initializers for union work in C++11?
According to doc, it is C++20 feature
https://en.cppreference.com/w/cpp/language/aggregate_initialization
union u { int a; const char* b; };
// C++20 designated initializer lists
u d = {.b = "asdf"}; // OK: can explicitly initialize a non-initial member
But I can compile this
❯ cat union.cc
union U{
int a;
float b;
};
int main(){
auto u = U{.a=3};
return 0;
}
~/code/des_init
❯ gcc ./union.cc -std=c++11
>Solution :
C added designated initializers to the language. Many C++ compilers implemented it in C, and in non-strict conformance mode allowed C++ programs to use the feature.
It was added to C++ with a few restrictions later (such as a requirement to do the initializers in order).
Tell your compiler to use strict conformance to block some or all extensions to the language they include by default.
-pedantic and -Werror and/or -pedantic-errors are all ways to block various extensions.
See How to disable GNU C extensions? for someone talking about this from the perspective of C; many of the same flags work in C++.
gcc by default seeks to compile valid programs, but does not guarantee to reject all invalid programs.