137 lines
3.9 KiB
Markdown
137 lines
3.9 KiB
Markdown
# Chapter 5. Statements
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## 5.1 Simple Statements
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## 5.2 Statement Scope
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## 5.3 Conditional Statements
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## 5.4 Iterative Statements
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### 5.4.1 The `while` Statement
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### 5.4.2 Traditional for Statement
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### 5.4.3 Range `for` Statement (C++11)
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**range for statement**: simpler `for` statement that used to iterate through elements of an container
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Syntax:
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```cpp
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for (declaration : expression)
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statement
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```
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* where *expression* must be a sequence
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* *declaration* defines a variable. Every element in sequence must be converted into declaration
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* Hence, `auto` type specifier is best
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```cpp
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vector<int> v = {0,1,2,3,4,5,6,7,8,9};
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// range variable must be a reference so we can write to the elements
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for (auto &r : v) // for each element in v, as we want to change r, we declare it as reference
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r *= 2; // double the value of each element in v
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```
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which equivalent to
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```cpp
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for (auto beg = v.begin(), end = v.end(); beg != end; ++beg) {
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auto &r = *beg; // r must be a reference so we can change the element
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r *= 2; // double the value of each element in v
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}
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```
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Q: Why cannot use a range `for` to add elements to a `vector`
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A: In a range `for`, the value of `end()` is cached. If add elements to sequence, the value of `end()` will be invalidated.
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### 5.4.4 The do while Statement
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## 5.5 Jump Statements
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### 5.5.1 The `break` Statement
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### 5.5.2 The `continue` Statement
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### 5.5.3 The `goto` Statement
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???
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## 5.6 `try` Blocks and Exception Handling
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* **throw expressions**: the part which the detecting part uses to indicate that it encountered sth run-time error
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* **try blocks**: code blocks are run, from where exception may occue
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* starts with `try` and ends with multiple **catch clause**
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* Exceptions thrown from code executed inside a `try` block are handled by one of `cath` clauses.
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* `catch` clauses are also called **exception handler**
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### 5.6.1 A `throw` Expression
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Syntax of `throw` Expression: keyword `throw` followed by an expression.
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e.g. convert following to code snippet with `throw`
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```cpp
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Sales_item item1, item2;
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cin >> item1 >> item2;
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// first check that item1 and item2 represent the same book
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if (item1.isbn() == item2.isbn()) {
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cout << item1 + item2 << endl;
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return 0; // indicate success
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} else {
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cerr << "Data must refer to same ISBN"
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<< endl;
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return -1; // indicate failure
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}
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```
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equivalent with throw expression. Rewrite the test to throw an exception rather than returning an error indicator
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```cpp
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// first check that the data are for the same item
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if (item1.isbn() != item2.isbn())
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throw runtime_error("Data must refer to same ISBN");
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// if we're still here, the ISBNs are the same
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cout << item1 + item2 << endl;
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```
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* Throwing an exception terminates the current function and transfers control to a handler that know how to handle this error.
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* `runtime_error` is one of std library exception types, defined in `stdexcept` header.
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### 5.6.2 The `try` Block
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general form of `try` block
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```cpp
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try {
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program-statements
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} catch (exception-declaration) {
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handler-statements
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} catch (exception-declaration) {
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handler-statements
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} // . . .
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```
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???
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### 5.6.3 Standard Exceptions
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Exception classed defined in std library, defined in 4 headers:
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1. `exception` header: most general exception class.
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1. Only show exception occur, no additional info
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2. `stdexcept` header: some general-purpose exception classes, as shown in table
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3. `new` header: `bad_alloc` exception type, detailed in Chapter 12.1.2
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4. `type_info` header: `bad_cast` exception type, detailed in Chapter 19.2
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| std exception classes in `<stdexcept>` | Description |
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| -------------------------------------- | -------------------------------- |
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| `exception` | Most general kind of problem |
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| `runtime_error` | Problem detected only at runtime |
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| `range_error` | Run-time error: ??? |
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??? Not finished ??? |