C++ template instance as function parameter and lambda binary predicate - c++

Question

I am in the process of fixing some of the sonarcloud issues on my project. One complaint is the function where I read and deserialize the settings for my application. Basically there are 50 or so blocks of this in one function:
if (!m_logLevel.deserializeFromSettings(settings, HGGW_KEY_LOGLEVEL) || m_logLevel.value() <= 0 || m_logLevel.value() > 4)
{
m_logLevel.update(HGGW_DEFAULT_LOGLEVEL, 0);
m_logLevel.serializeToQSettings(settings, HGGW_KEY_LOGLEVEL);
}
The members are instances of a templated class:
template <typename T>
class AwsProperty
{
public:
AwsProperty(T value, qint64 timestamp) : m_timestamp(timestamp), m_data(value){}
//other stuff, not important
}
//example:
AwsProperty<int> m_property;
I tried to design this function:
template <typename X>
void readAWSSettingOrPopulateDefault(AwsProperty<X> property, QString key, X default_val, QSettings settings, bool (*predicate)(X)){
if(!property.deserializeFromSettings(settings,key) || predicate(property.value())){
property.update(default_val, 0);
property.serializeToQSettings(settings, key);
}
}
Trying to call it:
this->readAWSSettingOrPopulateDefault(m_logLevel, HGGW_KEY_LOGLEVEL, HGGW_DEFAULT_LOGLEVEL, settings, [](int val){return val <= 0 || val > 4;});
Yields this error:
XXXXconfig.cpp:275: error: no matching function for call to ‘LocalConfig::readAWSSettingOrPopulateDefault(HGGWAwsPropertyInt32&, const QString&, const int&, QSettings&, LocalConfig::init(QString)::<lambda(int)>)’
this->readAWSSettingOrPopulateDefault(m_logLevel, HGGW_KEY_LOGLEVEL, HGGW_DEFAULT_LOGLEVEL, settings, [](int val){return val <= 0 || val > 4;});
XXXXconfig.cpp:275: note: mismatched types ‘bool (*)(X)’ and ‘LocalConfig::init(QString)::<lambda(int)>’
this->readAWSSettingOrPopulateDefault(m_logLevel, HGGW_KEY_LOGLEVEL, HGGW_DEFAULT_LOGLEVEL, settings, [](int val){return val <= 0 || val > 4;});
^ ^
XXXXconfig.cpp:275: note: mismatched types ‘bool (*)(X)’ and ‘LocalConfig::init(QString)::<lambda(int)>’
^

Answer 1

The reason your code reports an error is that for a function template declared as:
template <typename X>
void readAWSSettingOrPopulateDefault(AwsProperty<X> property, QString key
, X default_val, QSettings settings
, bool (*predicate)(X));
the compiler will have to deduce X from all arguments using this template parameter -- property, default_val and predicate -- and then make sure that those deduced types are the same (that is, there's only one template parameter, it can hold one type).
However, a lambda is not a function. It is convertible to a function pointer through an implicit conversion. Template argument deduction does not consider conversions (except for the derived-to-base conversion). Hence, the compiler is unable to finalize template argument deduction.
In order to make your code work, put the last occurrence of X in the function pointer-type parameter declaration in a non-deduced context:
template <typename T>
struct dont_deduce { using type = T; };
template <typename T>
using dont_deduce_t = typename dont_deduce<T>::type;
template <typename X>
void readAWSSettingOrPopulateDefault(AwsProperty<X> property, QString key
, X default_val, QSettings settings
, bool (*predicate)(dont_deduce_t<X>));
This way, the compiler will not even attempt to deduce X from the lambda expression, and instead will use the type from property/default_val, thus making it possible to use the implicit conversion of the lambda to a function pointer. That is, no deduction and at the same time, a known function signature that can trigger the conversion.