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This diagnostic rule is based on the MISRA (Motor Industry Software Reliability Association) software development guidelines.
This diagnostic rule is relevant only for C++.
Applying std::move to an object of the const T type produces a value of the const T&& type. Under ideal passing conditions, this type will not invoke a constructor or move operator, since their signatures require the T&& mutable rvalue reference. According to the overload resolution rules, the compiler selects the copy constructor (if one is available). As a result, the object is copied rather than moved, which leads to an unintended copy of its resources.
The example:
void archiveData()
{
std::vector<std::string> archive;
const std::string technicalId = ....;
// ....
archive.push_back(std::move(technicalId));
}The technicalId variable is marked with the const qualifier, meaning that the result of std::move(technicalId) will have the const std::string&& type. So, when resolving the overload for std::vector<std::string>::push_back, the version that takes const std::string & and results in a copy of the string will be selected.
The fix may look like this:
Option N1. If omitting the constness of the object being moved is not possible, the call to std::move needs to be removed:
void archiveData()
{
std::vector<std::string> archive;
const std::string technicalId = ....;
// ....
archive.push_back(technicalId);
}Option N2. The object constness can be omitted:
void archiveData()
{
std::vector<std::string> archive;
std::string technicalId = ....;
// ....
archive.push_back(std::move(technicalId));
}Removing the const qualifier changes the resulting expression type to std::string &&. This enables the compiler to successfully select the std::vector<std::string>::push_back overload that takes std::string &&, which then takes the resources from the passed object.
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