template function in contract code with a lambda

Question

This is more a C++ question than EOS contracts question but I am having trouble creating a template function for migrating tables to a different scope. I cannot get this to compile.

template <typename T, typename E, typename F>
void migrate_table(name code, uint16_t batch_size, name source_scope, name destn_scope, void (*mapper)(E&, F&) ) {
    T source(code, source_scope.value);
    T destination(code, destn_scope.value);
    auto lastdest = destination.end();
    if (lastdest == destination.begin()) {
        return; // empty table nothing copy
    }
    uint64_t src_primary_key = (--lastdest)->primary_key();

    auto source_itrr = source.find(src_primary_key);
    source_itrr++;
    uint16_t batch_counter = 0;

    while (batch_counter < batch_size && source_itrr != source.end()) {
        destination.emplace(code, [&](E &e){
            mapper(e, source_itrr);
        });
        ++source_itrr;
        ++batch_counter;
    }
} 

when I am trying to call the function like this:

    migrate_table<regmembers>(get_self(), batch_size, get_self(), NEW_SCOPE, [](member& dest, member& src){
            dest.sender = src.sender;
            dest.agreedtermsversion = src.agreedtermsversion;
        });

The compilation error I get is:

no matching function for call to 'migrate_table' migrate_table(get_self(), batch_size, get_self(), NEW_SCOPE, [](member& dest, member& src){ ^~~~~~~~~~~~~~~~~~~~~~~~~ ... candidate template ignored: could not match 'void (*)(E &, F &)' against '(lambda at

void migrate_table(name code, uint16_t batch_size, name source_scope, name destn_scope, void (*mapper)(E&, F&) ) {

I am guessing is something about the way I am passing the parameter as a reference and it being a template type but I cannot get it to work. I could avoid the template aspect all together but I have quite a few tables to migrate so would like to avoid the duplicate code.

Answer 1

template <typename T, typename Lambda>
void migrate_table(name code, uint16_t batch_size, name source_scope, name destn_scope, Lambda&& mapper) {
    T source(code, source_scope.value);
    T destination(code, destn_scope.value);
    auto lastdest = destination.end();
    if (lastdest == destination.begin()) {
        return; // empty table nothing copy
    }
    uint64_t src_primary_key = (--lastdest)->primary_key();

    auto source_itrr = source.find(src_primary_key);
    source_itrr++;
    uint16_t batch_counter = 0;

    while (batch_counter < batch_size && source_itrr != source.end()) {
        destination.emplace(code, [&](auto &e){
            mapper(e, *source_itrr); // source_itrr is iterator, so pass it dereferenced
        });
        ++source_itrr;
        ++batch_counter;
    }
}

    migrate_table<regmembers>(get_self(), batch_size, get_self(), NEW_SCOPE, [](member& dest, member const& src){ // dereferenced `src` is const object
        dest.sender = src.sender;
        dest.agreedtermsversion = src.agreedtermsversion;
    });