{"id":1989,"date":"2025-01-26T21:29:14","date_gmt":"2025-01-26T21:29:14","guid":{"rendered":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/?page_id=1989"},"modified":"2025-01-27T18:49:50","modified_gmt":"2025-01-27T18:49:50","slug":"unexpected-exceptions","status":"publish","type":"page","link":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/topics\/c-details\/exceptions-2\/unexpected-exceptions\/","title":{"rendered":"Unexpected Exceptions"},"content":{"rendered":"\n

<\/span>Unexpected Exceptions<\/span><\/h2>\n\n\n\n

Careless use of exception specifications can lead to calls to unexpected()<\/code><\/strong>.<\/p>\n\n\n\n

Such calls can be avoided through careful organization of exceptions and specifications.<\/p>\n\n\n\n

For example, a well-defined subsystem, say DBAccess<\/em>, will often have all its exceptions be derived from a class DBAccessException<\/code><\/strong>:<\/p>\n\n\n\n

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class<\/span> <\/span>DBAccessException<\/span> <\/span><\/span>\n{ <\/span><\/span>\n  \/* ... *\/<\/span> <\/span><\/span>\n};<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>DBConstraintViolation<\/span> : <\/span>public<\/span> <\/span>DBAccessException<\/span><\/span>\n{ <\/span><\/span>\n  \/* ... *\/<\/span> <\/span><\/span>\n};<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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which can be thrown from a function:<\/p>\n\n\n\n
void getDBData()<\/strong><\/code><\/pre>\n\n\n\n
for instance.<\/p>\n<\/div>\n<\/div>\n\n\n\n
Alternatively, calls to unexpected()<\/code><\/strong> can be intercepted by using the set_unexpected()<\/code><\/strong> call.<\/p>\n\n\n\n
For example:<\/p>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Intercepting Unexpected Exceptions<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/7\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span><exception><\/span><\/span>\n<\/span>\n\/\/ Exception classes<\/span><\/span>\nclass<\/span> <\/span>HissyFit<\/span> {};<\/span><\/span>\nclass<\/span> <\/span>Up<\/span> {};<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>my_unexpected_handler<\/span>() <\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cerr << <\/span>"Unexpected exception occurred!<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n  <\/span><\/span>\n  <\/span>exit<\/span>(<\/span>1<\/span>);<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doThat<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cerr << <\/span>"In doThat()...<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n  <\/span>throw<\/span> <\/span>42<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doThis<\/span>( <\/span>int<\/span> <\/span>i<\/span> ) <\/span>throw<\/span> (<\/span>HissyFit<\/span>, <\/span>Up<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cerr << <\/span>"In doThis()...<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n<\/span>\n  <\/span>switch<\/span> (i)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>case<\/span> <\/span>1<\/span>:<\/span><\/span>\n      <\/span>throw<\/span> <\/span>HissyFit<\/span>();<\/span><\/span>\n    <\/span>case<\/span> <\/span>2<\/span>:<\/span><\/span>\n      <\/span>throw<\/span> <\/span>Up<\/span>();<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>std<\/span>::cerr << <\/span>"In doThat() after switch statement...<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n<\/span>\n  <\/span>doThat<\/span>();<\/span><\/span>\n    \/\/ Throws an exception not in the doThis exception specification<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>(<\/span>int<\/span> <\/span>argc<\/span>, <\/span>const<\/span> <\/span>char<\/span> <\/span>*<\/span> <\/span>argv<\/span>[]) <\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::<\/span>set_unexpected<\/span>( my_unexpected_handler );<\/span><\/span>\n<\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>1<\/span>; i <=<\/span>3<\/span>; i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>try<\/span><\/span>\n    {<\/span><\/span>\n      <\/span>doThis<\/span>(i);<\/span><\/span>\n    }<\/span><\/span>\n    <\/span>catch<\/span> (HissyFit e)<\/span><\/span>\n    {<\/span><\/span>\n      <\/span>std<\/span>::cerr << <\/span>"Caught HissyFit<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n    }<\/span><\/span>\n    <\/span>catch<\/span> (Up)<\/span><\/span>\n    {<\/span><\/span>\n      <\/span>std<\/span>::cerr << <\/span>"Caught Up<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n    }<\/span><\/span>\n    <\/span>catch<\/span> (...)<\/span><\/span>\n    {<\/span><\/span>\n      <\/span>std<\/span>::exception_ptr eptr = <\/span>std<\/span>::<\/span>current_exception<\/span>();<\/span><\/span>\n      <\/span>try<\/span><\/span>\n      {<\/span><\/span>\n        <\/span>if<\/span> (eptr)<\/span><\/span>\n        {<\/span><\/span>\n          <\/span>std<\/span>::<\/span>rethrow_exception<\/span>(eptr);<\/span><\/span>\n        }<\/span><\/span>\n      }<\/span><\/span>\n      <\/span>catch<\/span> (<\/span>const<\/span> <\/span>std<\/span>::exception& e)<\/span><\/span>\n      {<\/span><\/span>\n        <\/span>std<\/span>::cerr << <\/span>"Caught exception: "<\/span> << <\/span>e<\/span>.<\/span>what<\/span>() << <\/span>std<\/span>::endl;<\/span><\/span>\n      }<\/span><\/span>\n    }<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>return<\/span> <\/span>0<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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… which outputs the following:<\/p>\n\n\n\n
In doThis()...\nCaught HissyFit\nIn doThis()...\nCaught Up\nIn doThis()...\nIn doThat() after switch statement...\nIn doThat()...\nUnexpected exception occurred!\nProgram ended with exit code: 1<\/strong><\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
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However…<\/p>\n\n\n\n
The above program worked in C++11. Unfortunately, in C++17, the exception specifications were deprecated, which meant that the set_unexpected()<\/code><\/strong> function is also deprecated.<\/p>\n<\/blockquote>\n\n\n\n
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See what happens with the above program in C++17:<\/p>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Intercepting Unexpected Exceptions<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/7\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span><exception><\/span><\/span>\n<\/span>\n\/\/ Exception classes<\/span><\/span>\nclass<\/span> <\/span>HissyFit<\/span> {};<\/span><\/span>\nclass<\/span> <\/span>Up<\/span> {};<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>my_unexpected_handler<\/span>() <\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cerr << <\/span>"Unexpected exception occurred!<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n  <\/span><\/span>\n  <\/span>exit<\/span>(<\/span>1<\/span>);<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doThat<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cerr << <\/span>"In doThat()...<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n  <\/span>throw<\/span> <\/span>42<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doThis<\/span>( <\/span>int<\/span> <\/span>i<\/span> ) <\/span>throw<\/span> (<\/span>HissyFit<\/span>, <\/span>Up<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cerr << <\/span>"In doThis()...<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n<\/span>\n  <\/span>switch<\/span> (i)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>case<\/span> <\/span>1<\/span>:<\/span><\/span>\n      <\/span>throw<\/span> <\/span>HissyFit<\/span>();<\/span><\/span>\n    <\/span>case<\/span> <\/span>2<\/span>:<\/span><\/span>\n      <\/span>throw<\/span> <\/span>Up<\/span>();<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>std<\/span>::cerr << <\/span>"In doThat() after switch statement...<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n<\/span>\n  <\/span>doThat<\/span>();<\/span><\/span>\n    \/\/ Throws an exception not in the doThis exception specification<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>(<\/span>int<\/span> <\/span>argc<\/span>, <\/span>const<\/span> <\/span>char<\/span> <\/span>*<\/span> <\/span>argv<\/span>[]) <\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::<\/span>set_unexpected<\/span>( my_unexpected_handler );<\/span><\/span>\n<\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>1<\/span>; i <=<\/span>3<\/span>; i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>try<\/span><\/span>\n    {<\/span><\/span>\n      <\/span>doThis<\/span>(i);<\/span><\/span>\n    }<\/span><\/span>\n    <\/span>catch<\/span> (HissyFit e)<\/span><\/span>\n    {<\/span><\/span>\n      <\/span>std<\/span>::cerr << <\/span>"Caught HissyFit<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n    }<\/span><\/span>\n    <\/span>catch<\/span> (Up)<\/span><\/span>\n    {<\/span><\/span>\n      <\/span>std<\/span>::cerr << <\/span>"Caught Up<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n    }<\/span><\/span>\n    <\/span>catch<\/span> (...)<\/span><\/span>\n    {<\/span><\/span>\n      <\/span>std<\/span>::exception_ptr eptr = <\/span>std<\/span>::<\/span>current_exception<\/span>();<\/span><\/span>\n      <\/span>try<\/span><\/span>\n      {<\/span><\/span>\n        <\/span>if<\/span> (eptr)<\/span><\/span>\n        {<\/span><\/span>\n          <\/span>std<\/span>::<\/span>rethrow_exception<\/span>(eptr);<\/span><\/span>\n        }<\/span><\/span>\n      }<\/span><\/span>\n      <\/span>catch<\/span> (<\/span>const<\/span> <\/span>std<\/span>::exception& e)<\/span><\/span>\n      {<\/span><\/span>\n        <\/span>std<\/span>::cerr << <\/span>"Caught exception: "<\/span> << <\/span>e<\/span>.<\/span>what<\/span>() << <\/span>std<\/span>::endl;<\/span><\/span>\n      }<\/span><\/span>\n    }<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>return<\/span> <\/span>0<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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… which produces compilte-time errors:<\/p>\n\n\n\n
main.cpp:28:22 ISO C++17 does not allow dynamic exception specifications<\/strong>\n\nmain.cpp:48:8 No type named 'set_unexpected' in namespace 'std'<\/strong>\n\n<\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
<\/span>Uncaught Exceptions & terminate()<\/span><\/h3>\n\n\n\n
If an exception is thrown, but not caught by an exception handler, the function terminate()<\/code><\/strong> will be called.<\/p>\n\n\n\n
terminate()<\/code><\/strong> will also be called:<\/p>\n\n\n\n