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Template Placeholders

note

This chapter focuses on the binary encoding of template placeholders. The Macros section explains what they are and how they are used.

Template placeholders are special constructs used within macro template bodies to indicate where macro arguments should be substituted. They are encoded using opcodes 0xE8, 0xE9, and 0xEA.

Placeholders may only occur in value position, only within a macro body; they are illegal anywhere else.

Placeholder Types

OpcodePlaceholder Type
0xE8Tagged template placeholder (no default)
0xE9Tagged template placeholder (with default)
0xEATagless template placeholder

Tagged Template Placeholder with No Default

Opcode 0xE8 indicates a tagged template placeholder with no default value. No additional bytes follow.

Encoding of (:?)
┌──── Opcode 0xE8 indicates a tagged template placeholder with no default
E8
Encoding of foo::(:?)
      ┌── Annotation text: foo
┌─────┴──────┐ ┌──── Opcode 0xE8 indicates a tagged template placeholder with no default
59 07 66 6F 6F E8

Tagged Template Placeholder with Default

Opcode 0xE9 indicates a tagged template placeholder with a default value. The default value follows and may be any value or e-expression that produces a value. A NOP is legal, and ignored.

Encoding of (:? "foo")
┌──── Opcode 0xE9 indicates a tagged template placeholder with default
│   ┌─── Default value: string "foo"
E9  93 66 6F 6F
Encoding of (:? 42)
┌──── Opcode 0xE9 indicates a tagged template placeholder with default
│   ┌─── Default value: integer 42
E9  61 2A
Encoding of (:? 42) with NOP
┌──── Opcode 0xE9 indicates a tagged template placeholder with default
│   ┌─── NOP
│   │   ┌─── Default value: integer 42
E9  EC  61 2A
Encoding of (:? $10::false)
┌──── Opcode 0xE9 indicates a tagged template placeholder with default
│   
│     ┌──── Annotation SID: $10
│   ┌─┴─┐ ┌─── The annotated value: `false`
E9  58 15 6F
    └──┬───┘
    The default value: `$10::false`

Tagless Template Placeholder

Opcode 0xEA indicates a tagless template placeholder. A single byte follows indicating the tagless scalar type that the argument must conform to. No additional bytes follow.

Encoding of (:?\int8\)
┌──── Opcode 0xEA indicates a tagless template placeholder
│  ┌─── Tagless scalar type: int8 (0x61)
EA 61
Encoding of (:?\uint32\)
┌──── Opcode 0xEA indicates a tagless template placeholder
│  ┌─── Tagless scalar type: uint32 (0xE4)
EA E4
Encoding of (:?\string\)
┌──── Opcode 0xEA indicates a tagless template placeholder
│  ┌─── Tagless scalar type: string (0xF9)
EA F9