Exceptions¶
Exceptions¶
Run-time Error
The basic philosophy of C++ is that “badly formed code will not be run.
Exception 我们能预见他发生,但不是每次都发生。
read a file
- open the file;
- determine its size;
- allocate that much memory;
- read the file into memory;
- close the file;
可以预见的错误:找不到文件,文件虽然存在但是打不开(没有权限、被别的进程打开),文件大小判断失败(可能是一个串口,不是磁盘上的文件,串口是没有结束的)可以 fseek 到文件末尾,再 ftell 末尾的值。
errorCodeType readFile { initialize errorCode = 0;
open the file;
if ( theFilesOpen ) { determine its size;
if ( gotTheFileLength ) { allocate that much memory;
if ( gotEnoughMemory ) { read the file into memory;
if ( readFailed ) { errorCode = -1;
} } else {
errorCode = -2;
} } else {
errorCode = -3;
}
close the file;
if ( theFILEDidntClose && errorCode == 0 ) { errorCode = -4;
} else {
errorCode = errorCode and -4;
} } else {
errorCode = -5;
}
return errorCode;
}
try {
open the file;
determine its size;
allocate that much memory;
read the file into memory;
close the file;
}
catch ( fileOpenFailed ) { doSomething; }
catch ( sizeDeterminationFailed ) { doSomething; }
catch ( memoryAllocationFailed ) { doSomething; }
catch ( readFailed ) { doSomething; }
catch ( fileCloseFailed ) { doSomething; }
At the point where the problem occurs, you might not know what to do with it, but you do know that you can’t just continue on merrily; you must stop, and somebody, somewhere, must figure out what to do.
运行时刻影响程序运行的,能预见会发生但不是每一次发生的事件。
- The significant benefit of exceptions is that they clean up error handling code.
- It separates the code that describes what you want to do from the code that is executed.
Vector
template <class T> class Vector {
private:
T* m_elements;
int m_size;
public:
Vector (int size = 0) :
m_size(size) ...
~Vector () { delete [] m_elements; }
void length(int);
int length() { return m_size; }
T& operator[](int);
};
if (indx < 0 || indx >= m_size) {
// throw is a keyword
// exception is raised at this point
throw <<something>>;
}
try{}
的大括号都可以看作异常),然后往外走。上面这里 throw 之后,如果大括号是语句就离开语句,如果是函数就离开函数,如果是 try{}
, 我们就判断匹配。一个异常类
class VectorIndexError {
public:
VectorIndexError(int v) : m_badValue(v) { }
~VectorIndexError() { }
void diagnostic() {
cerr << "index " << m_ badValue << "out of range!"; }
private:
int m_badValue;
};
throw VectorIndexError(indx);
. 外部就会拿到这个对象。需要用 try catch 捕捉,否则会直接结束。捕捉之后整个 try catch 就执行结束。可以捕捉到异常后再继续 throw.
void outer2() {
String err("exception caught");
try {
func();
} catch (VectorIndexError) {
cout << err;
throw; // propagate the exception
}
}
catch(...)
表示捕捉所有异常,但这样无法获得异常的对象。
- Is that surrounding a trye
- NO: leave that scope
- Is it a method body
- YES: return to the caller, then
- NO: go to 1st line
- Is it a method body
- YES: try to match a catch
- YES: do the clause and
- NO:
- NO: leave that scope
发生异常离开 fun 的时候本地变量会被析构。
throw VIEE(idx)
的对象在堆里,但是不被清除,会一直留在原地. catch 捕捉到对象之后,结束 try catch 之后会把对象清除。(这里不会发生拷贝构造,仍然是刚刚堆里的对象)
throw;
reraises the exception being handled. valid only within a handler
Try blocks¶
try 后面可以跟任意数量的 catch.
Two forms
catch (SomeType v) { // handler code }
catch (...) { // handler code }
throw 可以抛的任意类型, int/double/... 也是可以的。一般不会抛原始数据类型,因为表达的信息有限。通常会做一个类,抛类的对象。
Handlers are checked in order of appearance
按顺序匹配,先匹配前面的,匹配成功后不会寻求下一个匹配。
- Check for exact match
- Apply base class conversions
Reference and pointer types, only
对象会进行基类转换。一般把子类放在前面。 - Ellipses (...) match all
Exceptions and new¶
new
does NOT returned 0 on failure. new
raises a bad_alloc()
exception.
Standard library exceptions
Exception specifications¶
Declare which exceptions function might raise
Part of function prototypes
e.g. void abc(int a) : throw(MathErr) {}
调了别人的函数,要为了别人抛异常做准备。
可以在函数头部声明会抛什么异常。不会在编译时期检查。
限制 abc
而非调用 abc
函数。如果抛出了比你声明更多的异常,异常检查机制会抛出一个终止程序的异常。
Printer::print(Document&) :
throw(PrinterOffLine, BadDocument)
{ ...
PrintManager::print(Document&) :
throw (BadDocument) { ...
// raises or doesn’t handle BadDocument
void goodguy() : throw () {
// handles all exceptions
void average() { } // no spec, no checking,
- 第一个表示会抛
PrinterOffLine, BadDocument
异常。(不一定抛,但可能) - 第三个表示不会抛任何异常,这样调用的时候不需要 try catch.
- 第四个可能会抛异常,但是编译器不会进行检查。
Design considerations¶
- Exceptions should indicate errors.
exception 不是 routine. - Don’t use exceptions in place of good design.
Summary
- Error recovery is a hard design problem
- All subsystems need help from their clients to handle exceptional cases
- Exceptions provide the mechanism
- Propagated dynamically
- Objects on stack destroyed properly
- Act to terminate the problematic function
- Another big use:
Constructors that can’t complete their work
More exceptions¶
Exceptions and constructors¶
如果在构造的时候发生了异常。
先分配内存,再执行构造。
创建日期: 2023年6月6日 22:27:27