I'm trying to throw an exception in my code if a vector that is created from user input is not sorted in either descending or ascending order.
using namespace std;
#include <iostream>
#include <vector>
#include <algorithm>
int main () {
vector <int> vec;
//Let user fill a vector with 12 integers.
//cout << "Please note that input data should be either increasing or decreasing." << endl;
int n = 0;
int size = 0;
while(size < 12) {
cout << "Type integer to add to the vector." << endl;
cin >> n;
vec.push_back(n);
++size;
}
//throw exception if unsorted
try {
if (!((is_sorted(vec.begin(), vec.end())) || (is_sorted(vec.end(), vec.begin())))) {
throw "Input was not sorted.";
}
}
catch(exception &error){
cerr << "Error: " << error.what() << endl;
}
}
I have not included the rest of the code, which searches for a particular number, because I am pretty sure that it is irrelevant to this issue. When the data filled into the vector is ascending or descending, everything is fine, but when I test the exception, I get, "terminate called after throwing an instance of 'char const*' Aborted" instead of my desired error message. I don't understand what is going on here. Is it something wrong with the way I'm handling exceptions or am I using the sort() function incorrectly?
In C++, all types are throwable and catchable, but you are only catching subclasses of std::exception.
The best fix to your code would be changing your throw statement to:
throw std::runtime_error("Input was not sorted.");
If you want to catch an exception, you should throw an exception, not a const char*.
See this answer: c++ exception : throwing std::string
You're throwing a const char* not an std::exception. So catch it as a const char*:
catch(const char* error) {
std::cout << "Error: " << error << "\n";
}
Or throw an std::exception.
Remember that you can throw many types and have many catch blocks, the one that will be invoked is the one that matches the type of the thrown exception.
Related
I'm following an example code from Programming Principles and Practice Using C++ in one of the example for exception it shows this snippet of code
int main()
{
try {
vector<int> v; // a vector of ints
for (int x; cin >> x; )
v.push_back(x); // set values
for (int i = 0; i <= v.size(); ++i) // print values
cout << "v[" << i << "] == " << v[i] << '\n';
}
catch (const out_of_range& e) {
cerr << "Oops! Range error\n";
return 1;
}
catch (...) { // catch all other exceptions
cerr << "Exception: something went wrong\n";
return 2;
}
}
From what I understand it is suppose to catch out_of_range error and output "Oops! Range error". However, the Visual Studio 2019 shows this instead.
can someone explain why it shows me this
Does “try-catch” catches run time error (especially Out of Range error)?
No, in C++ most run time errors lead to Undefined Behavior, not exceptions. Only errors which explicitly throw exceptions can be caught.
std::vector<T>::operator[] does not specify that it throws an exception when you access out of bounds, it is just Undefined Behavior and anything can happen. It can even appear to work. When I try it here, there isn't any visible error : https://godbolt.org/z/Pcv9Gn8M9
If you want an exception on out of bounds access, std::vector<T>::at() does throw std::out_of_range.
For your test you should use v.at(i) instead of v[i]. Try it here : https://godbolt.org/z/szKxhjxhx.
I am trying to catch bad allocation error. When input length will be in order of 10000000000000000000000 or something, then bad allocation error should come. I don't know why its not being caught.
Any help will be appreciated!
# include <vector>
# include <iostream>
using namespace std;
void length(int m)
{
vector<int> x;
try
{
x.resize(m);
}
catch(std::bad_alloc&)
{
cout << "caught bad alloc exception" << std::endl;
}
}
int main()
{
int l;
cout << "Length" ;
cin >> l ;
length(l);
return 0;
}
UPDATED:
When I am hard coding the value for input, then it is throwing an exception. I don't know why its working this way.
# include <vector>
# include <iostream>
using namespace std;
void length(int m)
{
vector<int> x;
try
{
x.resize(m);
}
catch(std::bad_alloc&)
{
cout << "caught bad alloc exception" << std::endl;
}
}
int main()
{
int m= 100000000000000000000;
length(m);
return 0;
}
You ought to write
if (!(cin >> l)){
// I could not read that into `l`
}
The lack of an exception being caught could be down to
Your int value being smaller than you think (perhaps some undefined wrap-around behaviour), and an exception is not thrown since the allocation is successful.
The allocation being lazy in the sense that the memory is not allocated until you actually use it.
If std::bad_alloc is thrown as an anonymous temporary then it will not be caught at your catch site. (Unless your naughty compiler allows non-const references to bind to anonymous temporaries, which some do as an extension). Write catch (const std::bad_alloc&) instead, and it will be caught there.
The maximum length of an integer type int is 2.147.483.647 . Are you sure you have actually used an higher number to test it?
You're passing Integer variable which has the limit.
Minimum value for a variable of type short: –32768
Maximum value for a variable of type short: 32767
The error which you will get from your code is std::length_error
To raise the bad allocation error dynamically you can try malloc() with incorrect size OR try below code.
#include <iostream>
#include <new>
int main()
{
try {
while (true) {
new int[100000000ul];
}
} catch (const std::bad_alloc& e) {
std::cout << "Allocation failed: " << e.what() << '\n';
}
}
</i>
No exception is thrown because the int that makes it through to your function void length(int m) is capped at its max value that is much less than vector::max_size(). Consider:
void length(int m)
{
cout << "m is: " << m << " which has a max value: " << numeric_limits<int>::max() << endl;
// ...
}
with the output:
Length10000000000000000000000
m is: 2147483647 and has a max value: 2147483647
I'm just learning how to use exceptions in C++ and have come across weird behavior in my "test" code. (excuse overly stupid questions like this one, please...it's NOT lack of research/effort, just lack of experience!) If I'm catching just the exception DivideByZero it works fine.
But introducing the second exception StupidQuestion makes the code not work exactly how I expected. How I wrote it below I thought it should take care of the DivideByZero exception if it needs to, and if not then check if StupidQuestion occurs, and if not just go back to the try clause and print the normal result. But if I input, say, a=3 and b=1, the program redirects to the DivideByZero try clause instead of the StupidQuestion one. The weird thing is, though, divide does seem to be throwing StupidQuestion (see via cout statement), but it's not catching right, as also seen by the absense of the cout statement.
#include <iostream>
#include <cstdlib>
using namespace std;
const int DivideByZero = 42;
const int StupidQuestion=1337;
float divide (int,int);
main(){
int a,b;
float c;
cout << "Enter numerator: ";
cin >> a;
cout << "Enter denominator: ";
cin >> b;
try{
c = divide(a,b);
cout << "The answer is " << c << endl;
}
catch(int DivideByZero){
cout << "ERROR: Divide by zero!" << endl;
}
catch(int StupidQuestion){
cout << "But doesn't come over here...?" << endl;
cout << "ERROR: You are an idiot for asking a stupid question like that!" << endl;
}
system("PAUSE");
}
float divide(int a, int b){
if(b==0){
throw DivideByZero;
}
else if(b==1){
cout << "It goes correctly here...?" << endl;
throw StupidQuestion;
}
else return (float)a/b;
}
I was wondering if it had something to do with the fact that DivideByZero and StupidQuestion were both of type int, so I changed the code to make StupidQuestion be of type char instead of int. (So: const char StupidQuestion='F'; and catch(char StupidQuestion) were really the only things changed from above) And it worked fine.
Why isn't the above code working when the two exceptions have the same type (int)?
Instead of this
catch(int DivideByZero) {
cout << "ERROR: Divide by zero!" << endl;
}
catch(int StupidQuestion) {
cout << "But doesn't come over here...?" << endl;
cout << "ERROR: You are an idiot for asking a stupid question like that!" << endl;
}
you are looking for
catch (int errval) {
if (errval == DivideByZero) {
cout << "ERROR: Divide by zero!" << endl;
}
else if (errval == StupidQuestion) {
cout << "ERROR: You are an idiot for asking a stupid question like that!" << endl;
}
else {
throw; // for other errors, keep searching for a handler
}
}
The variable name inside the catch clause is creating a new local variable, which has no relation to a global constant with the same name.
Also note that there will be no way to catch just one error number... but you can rethrow unknown errors as I show.
catch(int DivideByZero) { }
catch(int StupidQuestion) { }
Both catch blocks catch ints, they're just named differently. Only the first one can ever be entered, the second one is dead code.
When choosing a handler for an exception only type is taken into account, and neither values nor addresses (addresses of variables are not applicable here at all because of how exceptions work), also names of variables do not exist after compilation.
The first appropriate handler for the exception is always chosen.
Please look my answer to another question for details: https://stackoverflow.com/a/45436594/1790694
My OS is Win8
using Code::Blocks 12.10
I'm trying to get a handle on throwing and handling exceptions using an example from
Starting Out with C++ Early Objects Addison Wesley.
Here is the simple code I'm using:
// This program illustrates exception handling
#include <iostream>
#include <cstdlib>
using namespace std;
// Function prototype
double divide(double, double);
int main()
{
int num1, num2;
double quotient;
//cout << "Enter two integers: ";
//cin >> num1 >> num2;
num1 = 3;
num2 = 0;
try
{
quotient = divide(num1,num2);
cout << "The quotient is " << quotient << endl;
}
catch (char *exceptionString)
{
cout << exceptionString;
exit(EXIT_FAILURE); // Added to provide a termination.
}
cout << "End of program." << endl;
return 0;
}
double divide(double numerator, double denominator)
{
if (denominator == 0)
throw "Error: Cannot divide by zero\n";
else
return numerator/denominator;
}
The program will compile, and when I use two ints > 0 execution is normal. If I try to divide by 0 however, I get the following message:
terminate called after throwing an instance of 'char const*'
This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.
Process returned 255 (0xFF) execution time : 4.485 s
Press any key to continue.
I've looked at other examples, but have yet to find similar code to derive an answer from.
Any advice?
There's a compelling example in the C++ Standard, [except.throw]/1:
Example:
throw "Help!";
can be caught by a handler of const char* type:
try {
// ...
} catch(const char* p) {
// handle character string exceptions here
}
When you throw via throw "Error: Cannot divide by zero\n";, the expression after throw is a string literal, therefore of type array of n const char (where n is the length of the string + 1). This array type is decayed to a pointer [except.throw]/3, therefore the type of the object thrown is char const*.
Which types are catched by a handler (catch) is described in [except.handle]/3, and none of the cases apply here, i.e. the const char* is not catched by a handler of type char*.
So I have an upcoming assignment dealing with exceptions and using them in my current address book program that most of the homework is centered around. I decided to play around with exceptions and the whole try catch thing, and using a class design, which is what I will eventually have to do for my assignment in a couple of weeks. I have working code that check the exception just fine, but what I want to know, is if there is a way to standardize my error message function, (i.e my what() call):
Here s my code:
#include <iostream>
#include <exception>
using namespace std;
class testException: public exception
{
public:
virtual const char* what() const throw() // my call to the std exception class function (doesn't nessasarily have to be virtual).
{
return "You can't divide by zero! Error code number 0, restarting the calculator..."; // my error message
}
void noZero();
}myex; //<-this is just a lazy way to create an object
int main()
{
void noZero();
int a, b;
cout << endl;
cout << "Enter a number to be divided " << endl;
cout << endl;
cin >> a;
cout << endl;
cout << "You entered " << a << " , Now give me a number to divide by " << endl;
cin >> b;
try
{
myex.noZero(b); // trys my exception from my class to see if there is an issue
}
catch(testException &te) // if the error is true, then this calls up the eror message and restarts the progrm from the start.
{
cout << te.what() << endl;
return main();
}
cout <<endl;
cout << "The two numbers divided are " << (a / b) << endl; // if no errors are found, then the calculation is performed and the program exits.
return 0;
}
void testException::noZero(int &b) //my function that tests what I want to check
{
if(b == 0 ) throw myex; // only need to see if the problem exists, if it does, I throw my exception object, if it doesn't I just move onto the regular code.
}
What I would like to be able to do is make it so my what() function can return a value dependent on what type of error is being called on. So for instance, if I were calling up an error that looked a the top number,(a), to see if it was a zero, and if it was, it would then set the message to say that "you can't have a numerator of zero", but still be inside the what() function. Here's an example:
virtual const char* what() const throw()
if(myex == 1)
{
return "You can't have a 0 for the numerator! Error code # 1 "
}
else
return "You can't divide by zero! Error code number 0, restarting the calculator..."; // my error message
}
This obviously wouldn't work, but is there a way to make it so I'm not writing a different function for each error message?
Your code contains a lot of misconceptions. The short answer is yes, you can change what() in order to return whatever you want. But let's go step by step.
#include <iostream>
#include <exception>
#include <stdexcept>
#include <sstream>
using namespace std;
class DivideByZeroException: public runtime_error {
public:
DivideByZeroException(int x, int y)
: runtime_error( "division by zero" ), numerator( x ), denominator( y )
{}
virtual const char* what() const throw()
{
cnvt.str( "" );
cnvt << runtime_error::what() << ": " << getNumerator()
<< " / " << getDenominator();
return cnvt.str().c_str();
}
int getNumerator() const
{ return numerator; }
int getDenominator() const
{ return denominator; }
template<typename T>
static T divide(const T& n1, const T& n2)
{
if ( n2 == T( 0 ) ) {
throw DivideByZeroException( n1, n2 );
}
return ( n1 / n2 );
}
private:
int numerator;
int denominator;
static ostringstream cnvt;
};
ostringstream DivideByZeroException::cnvt;
In the first place, runtime_error, derived from exception, is the adviced exception class to derive from. This is declared in the stdexcept header. You only have to initialize its constructor with the message you are going to return in the what() method.
Secondly, you should appropriately name your classes. I understand this is just a test, but a descriptive name will always help to read and understand your code.
As you can see, I've changed the constructor in order to accept the numbers to divide that provoked the exception. You did the test in the exception... well, I've respected this, but as a static function which can be invoked from the outside.
And finally, the what() method. Since we are dividing two numbers, it would be nice to show that two numbers that provoked the exception. The only way to achieve that is the use of ostringstream. Here we make it static so there is no problem of returning a pointer to a stack object (i.e., having cnvt a local variable would introduce undefined behaviour).
The rest of the program is more or less as you listed it in your question:
int main()
{
int a, b, result;
cout << endl;
cout << "Enter a number to be divided " << endl;
cout << endl;
cin >> a;
cout << endl;
cout << "You entered " << a << " , Now give me a number to divide by " << endl;
cin >> b;
try
{
result = DivideByZeroException::divide( a, b );
cout << "\nThe two numbers divided are " << result << endl;
}
catch(const DivideByZeroException &e)
{
cout << e.what() << endl;
}
return 0;
}
As you can see, I've removed your return main() instruction. It does not make sense, since you cannot call main() recursively. Also, the objective of that is a mistake: you'd expect to retry the operation that provoked the exception, but this is not possible, since exceptions are not reentrant. You can, however, change the source code a little bit, to achieve the same effect:
int main()
{
int a, b, result;
bool error;
do {
error = false;
cout << endl;
cout << "Enter a number to be divided " << endl;
cout << endl;
cin >> a;
cout << endl;
cout << "You entered " << a << " , Now give me a number to divide by " << endl;
cin >> b;
try
{
result = DivideByZeroException::divide( a, b ); // trys my exception from my class to see if there is an issue
cout << "\nThe two numbers divided are " << result << endl;
}
catch(const DivideByZeroException &e) // if the error is true, then this calls up the eror message and restarts the progrm from the start.
{
cout << e.what() << endl;
error = true;
}
} while( error );
return 0;
}
As you can see, in case of an error the execution follows until a "proper" division is entered.
Hope this helps.
You can create your own exception class for length error like this
class MyException : public std::length_error{
public:
MyException(const int &n):std::length_error(to_string(n)){}
};
class zeroNumerator: public std::exception
{
const char* what() const throw() { return "Numerator can't be 0.\n"; }
};
//...
try
{
myex.noZero(b); // trys my exception from my class to see if there is an issue
if(myex==1)
{
throw zeroNumerator(); // This would be a class that you create saying that you can't have 0 on the numerator
}
}
catch(testException &te)
{
cout << te.what() << endl;
return main();
}
You should always use std::exception&e. so do
catch(std::exception & e)
{
cout<<e.what();
}
You should consider a hierarchy of classes.
The reason for it might not be obvious when trying to use exceptions just for transferring a string, but actual intent of using exceptions should be a mechanism for advanced handling of exceptional situations. A lot of things are being done under the hood of C++ runtime environment while call stack is unwound when traveling from 'throw' to corresponded 'catch'.
An example of the classes could be:
class CalculationError : public std::runtime_error {
public:
CalculationError(const char * message)
:runtime_error(message)
{
}
};
class ZeroDeviderError : public CalculationError {
public:
ZeroDeviderError(int numerator, const char * message)
: CalculationError(message)
, numerator (numerator)
{
}
int GetNumerator() const { return numerator; }
private:
const int numerator;
};
Providing different classes for the errors, you give developers a chance to handle different errors in particular ways (not just display an error message)
Providing a base class for the types of error, allows developers to be more flexible - be as specific as they need.
In some cases, they might want to be specific
} catch (const ZeroDividerError & ex) {
// ...
}
in others, not
} catch (const CalculationError & ex) {
// ...
}
Some additional details:
You should not create objects of your exceptions before throwing in the manner you did. Regardless your intention, it is just useless - anyway, you are working with a copy of the object in the catch section (don't be confused by access via reference - another instance of the exception object is created when throwing)
Using a const reference would be a good style catch (const testException &te) unless you really need a non-constant object.
Also, please note that the type (classes) used for exceptions are not permitted to throw exceptions out of their copy constructors since, if the initial exception is attempted to be caught by value, a call of copy constructor is possible (in case is not elided by the compiler) and this additional exception will interrupt the initial exception handling before the initial exception is caught, which causes calling std::terminate.
Since C++11 compilers are permitted to eliminate the copying in some cases when catching, but both the elision is not always sensible and, if sensible, it is only permission but not obligation (see https://en.cppreference.com/w/cpp/language/copy_elision for details; before C++11 the standards of the language didn’t regulate the matter).
Also, you should avoid exceptions (will call them the additional) to be thrown out of constructors and move constructors of your types (classes) used for exceptions (will call them initial) since the constructors and move constructors could be called when throwing objects of the types as initial exceptions, then throwing out an additional exception would prevent creation of an initial exception object, and the initial would just be lost. As well as an additional exception from a copy constructor, when throwing an initial one, would cause the same.