Unintentional expensive copy should be avoided when using auto as a placeholder type.
When using const auto as a placeholder type, you might unintentionally forget to add an ampersand(&) after the
auto keyword. This can silently create a pointless copy and possibly have a bad impact on the performance of your code depending on the
size of the created object and its context.
For example, if it happens in a range-based for loop context, it is going to lead to creating as many useless objects as the size of the range.
This rule will detect a declaration of an unmodified local variable with expensive to copy type and auto as a placeholder type that is
initialized with a non-temporary object.
Noncompliant code example
void printVec(const std::vector<std::string>& namesOfTheEntirePopulation) {
for (const auto name : namesOfTheEntirePopulation) { // Noncompliant
std::cout << name;
}
}
void ignore(const std::vector<std::string>& vec);
void ignoreAgain(const std::vector<std::string>& vec);
void ignore(VecWrapper vec) {
const auto namesOfCPPHaters = vec.getNamesOfCPPHaters(); // Noncompliant
ignore(namesOfCPPHaters);
ignoreAgain(namesOfCPPHaters);
}
Compliant solution
void modifyName(std::string& a);
void printVec(std::vector<std::string>& namesOfTheEntirePopulation) {
for (const auto& name : namesOfTheEntirePopulation) { // Compliant
std::cout << name;
}
for (auto name : namesOfTheEntirePopulation) { // Compliant: a copy is needed to avoid modifying the original list of names
modifyName(name);
std::cout << name;
}
}
void ignore(const std::vector<std::string>& vec);
void ignoreAgain(const std::vector<std::string>& vec);
void ignore(VecWrapper vec) {
const auto& namesOfCPPHaters = vec.getNamesOfCPPHaters(); // Compliant
ignore(namesOfCPPHaters);
ignoreAgain(namesOfCPPHaters);
}
