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初识exception
2022-06-28 13:21:00 【GarryLau】
异常可抛出的类型（事实上，可以抛出任何类型）：
1.基本类型及复合类型;
2.类类型
使用(类)对象作为异常抛出的好处：
1.对象的类名称可以传递信息;
2.对象可以存储信息,包括用于描述异常的字符串.
(备注:建议按const引用捕获异常)
如果存在未捕获的异常,会调用内建的terminate()函数,这个函数调用中的abort()终止程序
没有抛出列表的函数可以抛出任何异常。具有noexcept或者空抛出列表throw()的函数不能抛出任何异常。
throw是C++的关键字，这是抛出异常的唯一方法。
要想抛出异常，要么主动使用throw，要么是调用了其它包含throw的方法。
#pragma once
#include <iostream>
#include <stdexcept>
namespace test_exception {
auto testThrow() -> void {
int a = 10, b = 0;
try {
if(0 == b) {
throw std::runtime_error("divided 0");
}
else{
int c = a / b;
}
}
catch(std::runtime_error& e){
std::cerr << "exception occurs: " << e.what() << std::endl;
}
}
auto testWantThrow() -> void {
int a = 10, b = 0;
try {
int c = a / b; // try语句块里没有throw,因此不会抛出异常
}
catch(...){
std::cerr << "exception occurs." << std::endl;
}
}
auto main() -> int {
std::cout << "testing exception_errorprocess..." << std::endl;
testThrow();
testWantThrow();
std::cout << "------------------------------" << std::endl;
return 0;
}
}
输出：
1.异常示例
#pragma once
#include <stdexcept>
#include <exception>
#include <iostream>
namespace test_exception {
float divide1(float a, float b) {
if(b == 0){
//throw "divided by zero.";
//throw std::invalid_argument("Divide by zero");
//throw std::exception();
throw 5;
}
return a / b;
}
float divide2(float a, float b) noexcept/*throw()*/{
if(b == 0){
//throw "divided by zero.";
throw std::invalid_argument("std::invalid_argument, Divide by zero!");
//throw std::exception();
//throw 5;
}
return a / b;
}
#if 1
float divide3(float a, float b) throw(int, std::invalid_argument){
// 表示只可抛出int, std::invalid_argument两种类型的异常
if(b == 0){
//throw "divided by zero."; // 此种类型无法抛出
throw std::invalid_argument("std::invalid_argument, Divide by zero!"); // 可抛出此种类型
//throw std::exception(); // 此种类型无法抛出
//throw 5; // 可抛出此种类型
}
return a / b;
}
#endif
auto catchSample1() -> void {
int a = 10;
int b = 0;
try {
std::cout << divide1(a, b) << std::endl;
}
catch(const char* e){
std::cout << e << std::endl;
}
catch(const std::invalid_argument& e){
std::cout << e.what() << std::endl;
}
catch(const std::exception& e){
// 增加const属性不会影响匹配的目的,即此句与std::exception& e均可匹配std::exception类型的异常
std::cout << "+++" << e.what() << "+++" << std::endl;
}
catch(int e){
std::cout << "catching integer\n";
}
catch(...){
// 三个点时与所有异常类型匹配的通配符,可以使用这一技术确保捕获所有可能的异常
std::cout << "unknow exception" << std::endl;
}
}
auto catchSample2() -> void {
int a = 10;
int b = 0;
std::cout << "entering catchSample2()...............\n";
try {
std::cout << divide2(a, b) << std::endl;
}
catch(const std::invalid_argument& e){
std::cout << e.what() << std::endl;
}
catch(...){
std::cout << "unknow exception" << std::endl;
}
std::cout << "exiting catchSample2()...............\n";
}
auto main() -> void {
std::cout << "testing exception......" << std::endl;
catchSample1();
catchSample2();
std::cout << "--------------------------------------\n";
}
}
float divide2(float a, float b) noexcept定义成禁止抛出异常，因此当除以0（异常发生）时在catchSample2() 并不进行捕获，而是交由操作系统处理（一般是调用terminate()）。
以上程序输出：
__cplusplus: 201103
testing exception......
catching integer
entering catchSample2()...............
terminate called after throwing an instance of 'std::invalid_argument'
what(): std::invalid_argument, Divide by zero!
需要注意的是对于形如上述代码中float divide3(float a, float b) throw(int, std::invalid_argument)抛出列表C++17已不再支持，因此如果使用C++17标准编译会报如下错误：error: ISO C++17 does not allow dynamic exception specifications
# specify the C++ standard
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED True)
2.异常层次结构
在类层次结构中捕获异常时需要注意：
1.利用多态捕获异常时，一定要按引用捕获。如果按值捕获异常就可能发生截断，此时丢失对象的信息。
2.catch语句应该按照限制减弱的顺序出现（特殊放在前、一般放在后）。例如如果想捕获invalid_argument异常，那么就应该将invalid_argument放在exception前面；如果invalid_argument放在exception后面那么invalid_argument永远无法执行。详见下面两个示例：
#pragma once
#include <stdexcept>
#include <exception>
#include <iostream>
namespace test_exception {
float divide(float a, float b) {
if(b == 0){
throw std::invalid_argument("Divide by zero");
//throw std::exception();
}
return a / b;
}
auto catchSample() -> void {
int a = 10;
int b = 0;
try {
std::cout << divide(a, b) << std::endl;
}
catch(const std::invalid_argument& e){
std::cout << e.what() << std::endl;
}
catch(const std::exception& e){
std::cout << "+++" << e.what() << "+++" << std::endl;
}
catch(...){
std::cout << "unknow exception" << std::endl;
}
}
auto main() -> void {
std::cout << "testing exception......" << std::endl;
catchSample();
std::cout << "--------------------------------------\n";
}
}
执行结果如下：
__cplusplus: 201703
testing exception......
Divide by zero
--------------------------------------
The end.
#pragma once
#include <stdexcept>
#include <exception>
#include <iostream>
namespace test_exception {
float divide(float a, float b) {
if(b == 0){
throw std::invalid_argument("Divide by zero");
//throw std::exception();
}
return a / b;
}
auto catchSample() -> void {
int a = 10;
int b = 0;
try {
std::cout << divide(a, b) << std::endl;
}
catch(const std::exception& e){
std::cout << "+++" << e.what() << "+++" << std::endl;
}
catch(const std::invalid_argument& e){
std::cout << e.what() << std::endl;
}
catch(...){
std::cout << "unknow exception" << std::endl;
}
}
auto main() -> void {
std::cout << "testing exception......" << std::endl;
catchSample();
std::cout << "--------------------------------------\n";
}
}
执行结果如下：
__cplusplus: 201703
testing exception......
+++Divide by zero+++
--------------------------------------
The end.
3.自定义异常类
建议从标准的exception直接或间接地继承的方式实现自定义异常类。
当某段代码抛出一个异常的时候,被抛出的值或者对象被复制,
即通过使用复制构造函数从旧对象构造新对象;复制是必须的,
因为原始对象在堆栈中的位置较高,因此可能在异常被捕获之前超出作用域(因此会被销毁,其所占的内存会被回收)。因此,如果编写的类的对象作为异常抛出,对象必须能被复制,这意味着如果动态分配了内存,必须编写析构函数、复制构造函数以及赋值运算符。P296
提示：作为异常抛出的对象至少会按值复制一次，按引用捕获异常可以避免不必要的复制
#pragma once
#include <stdexcept>
#include <string>
#include <iostream>
#include <fstream>
namespace test_exception {
class FileError : public std::runtime_error {
public:
FileError(const std::string& filename): filename_(filename), msg_(""), std::runtime_error(""){
}
virtual const char* what() const noexcept override{
return msg_.c_str();}
protected:
std::string filename_, msg_;
};
class FileOpenError : public FileError {
public:
FileOpenError(const std::string& filename): FileError(filename){
msg_ = std::string("Unable to open ") + filename_;
}
};
class FileReadError : public FileError {
public:
FileReadError(const std::string& filename, int linenumber): FileError(filename), linenum_(linenumber){
msg_ = std::string("Exception occurs in file ") + filename_ + std::string(", line ") + std::to_string(linenum_);
}
protected:
int linenum_;
};
auto main() -> void {
std::cout << "testing exception......" << std::endl;
std::string filename("../doc/daisy.txt");
std::ifstream in_file(filename);
try {
if(in_file.fail()){
throw FileOpenError(filename);
}
std::string line;
int linenum = 0;
while(std::getline(in_file, line)){
linenum++;
std::cout << line << std::endl;
}
if(!in_file.eof()){
in_file.close();
throw FileReadError(filename, linenum);
}
}
catch(const FileOpenError& e){
std::cout << "Line: " << __LINE__ << ", " << e.what() << std::endl;
}
catch (const FileReadError& e){
std::cout << "Line: " << __LINE__ << ", " << e.what() << std::endl;
}
catch(...){
std::cout << "unknow error occurs." << std::endl;
}
in_file.close();
std::cout << "--------------------------------------\n";
}
}
daisy.txt中的内容：
D:/daisy/5547758_eea9edfd54_n.jpg
D:/daisy/5673551_01d1ea993e_n.jpg
D:/daisy/5673728_71b8cb57eb.jpg
D:/daisy/5794835_d15905c7c8_n.jpg
D:/daisy/5794839_200acd910c_n.jpg
上述代码输出：
__cplusplus: 201703
testing exception......
D:/daisy/5547758_eea9edfd54_n.jpg
D:/daisy/5673551_01d1ea993e_n.jpg
D:/daisy/5673728_71b8cb57eb.jpg
D:/daisy/5794835_d15905c7c8_n.jpg
D:/daisy/5794839_200acd910c_n.jpg
--------------------------------------
The end.
Reference
1.Marc Gregoire, Nicholas A. Solter, Scott J. Kleper. C++高级编程(第2版). 清华大学出版社,2012
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