CodeQL documentation

Catching by value

ID: cpp/catch-by-value
Kind: problem
Security severity: 
Severity: warning
Precision: very-high
   - efficiency
   - correctness
   - exceptions
Query suites:
   - cpp-security-and-quality.qls

Click to see the query in the CodeQL repository

Catching an exception by value will create a new local variable which is a copy of the originally thrown object. Creating the copy is slightly wasteful, but not catastrophic. More worrisome is the fact that if the type being caught is a strict supertype of the originally thrown type, then the copy might not contain as much information as the original exception.


The parameter to the catch block should have its type changed from T to T& or const T&.


void bad() {
  try {
    /* ... */
  catch(std::exception a_copy_of_the_thrown_exception) {
    // Do something with a_copy_of_the_thrown_exception

void good() {
  try {
    /* ... */
  catch(const std::exception& the_thrown_exception) {
    // Do something with the_thrown_exception


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