Trouble Understanding Vulkan Shader - glsl

Question

From the Vulkan Tutorial:
#version 450
#extension GL_ARB_separate_shader_objects : enable
out gl_PerVertex {
vec4 gl_Position;
};
vec2 positions[3] = vec2[](
vec2(0.0, -0.5),
vec2(0.5, 0.5),
vec2(-0.5, 0.5)
);
void main() {
gl_Position = vec4(positions[gl_VertexIndex], 0.0, 1.0);
}
Question 1: What does this designate?
out gl_PerVertex {
vec4 gl_Position;
};
Question 2: What explains the syntax vec2 positions[3] = vec2[](...)? To initialize the array, shouldn't the syntax be
vec2 positions[3] = {
vec2(0.0, -0.5),
vec2(0.5, 0.5),
vec2(-0.5, 0.5)
};
Is this shader-specific syntax, or can arrayType[](...) be used as a constructer in C++ also?

Answer 1

Question 1: What does this designate?
This
out gl_PerVertex {
vec4 gl_Position;
};
is an Output Interface Block. (See further GLSL Specification - 4.3.9 Interface Blocks)
Question 2: What explains the syntax vec2 positions3 = vec2? To initialize the array, shouldn't the syntax be ...
No
See Array constructors
Arrays can be constructed using array constructor syntax. In this case, the
type also contains the [] array notation:
const float array[3] = float[3](2.5, 7.0, 1.5);
See GLSL Specification - 4.1.11 Initializers:
If an initializer is a list of initializers enclosed in curly braces, the variable being declared must be a
vector, a matrix, an array, or a structure.
int i = { 1 }; // illegal, i is not a composite
....
All of the following declarations result in a compile-time error.
float a[2] = { 3.4, 4.2, 5.0 }; // illegal
....
If an initializer (of either form) is provided for an unsized array, the size of the array is determined by the
number of top-level (non-nested) initializers within the initializer. All of the following declarations create
arrays explicitly sized with five elements:
float a[] = float[](3.4, 4.2, 5.0, 5.2, 1.1);
float b[] = { 3.4, 4.2, 5.0, 5.2, 1.1 };
float c[] = a; // c is explicitly size 5
float d[5] = b; // means the same thing