AST
- C++ course
- Expert
- Lesson 38/47
- 5 min
Learn the Abstract Syntax Tree (AST) structure.
AST
The AST (Abstract Syntax Tree) is the tree the compiler builds after parsing the source code. Each node represents a syntax unit (expression, statement, declaration).
Example: a + b * c becomes a BinaryExpr tree; on the left is a, on the right another BinaryExpr (b * c). This structure lets the compiler process the meaning (type checking, optimization). The AST is an internal representation independent of the source.
AST nodes
Common AST node types are: Expr (expression — a value-producing unit), Stmt (statement — a side-effect unit), BinaryExpr (a two-operand operation), Literal (a constant value), VarDecl (a variable declaration).
The compiler uses the AST in three ways: checking (type checking, scope analysis), optimization (constant folding, dead code elimination) and code generation (from AST to assembly). These synchronized phases turn the source into an executable.
Practice Write the Expr struct
Write struct Expr { int value; };. In main define Expr e{42}; and write std::cout << e.value;.
Starter code
#include <iostream>
// Define the Expr struct here
int main() {
// Create an Expr instance and print its value
return 0;
}Sample solution
#include <iostream>
struct Expr {
int value;
};
int main() {
Expr e{42};
std::cout << e.value;
return 0;
}Expected output
42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the AST struct
struct BinaryExpr { Expr* left; Expr* right; std::string op; ____;struct BinaryExpr { Expr* left; Expr* right; std::string op; };Explanation: A struct definition ends with }.
2 What does an AST node represent?
struct Expr { int value; };- A machine instruction
- A syntax unit (expression, statement) Correct answer
- A memory cell
- A file
Explanation: Each AST node represents a syntax unit.
3 Match each node to its role
| Term | Meaning |
|---|---|
Expr node |
Expression node |
Stmt node |
Statement node |
BinaryExpr |
Two-operand operation |
Literal |
Constant value |