Define all C++ model constructors explicit

[datumbox]

Static analysis on the C++ code at csrc/models/ produces the following warning:
Clang-Tidy: Constructors that are callable with a single argument must be marked explicit to avoid unintentional implicit conversions

By allowing implicit calls the following misleading usage is possible:

This can be stopped by defining all the constructors that receive up to 1 mandatory parameter as explicit:

After the change, the following 2 initializations are valid:

DenseNetImpl x1(100);
DenseNetImpl x2{100};

Note that thought this might be considered theoretically a breaking change on the C++ API, I believe not many users use the old initialization as it's quite misleading.

I tested the changes by manually running the test_cpp_models.py unit-tests. All pass except of test_mnasnet0_5, test_mnasnet0_75 and test_mnasnet1_3 which also fail on master.

[vfdev-5]

With explicit specifier we prevent of using copy initialization: no more of

DenseNetImpl net = 1000; // error: copy-list-initialization

[codecov]

Codecov Report

Merging #2944 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2944   +/-   ##
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  Coverage   73.41%   73.41%           
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  Files          99       99           
  Lines        8801     8801           
  Branches     1389     1389           
=======================================
  Hits         6461     6461           
  Misses       1915     1915           
  Partials      425      425           

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[datumbox]

@vfdev-5 we could still do:

DenseNetImpl z{100};
DenseNetImpl x = z;

But as you said this will no longer be not allowed:

DenseNetImpl x = 100;

More specifically what we don't allow the compiler to do is silently take the integer 100, pass it on the constructor as first argument to create a temporary object and then copy to x (thought optimizations can happen here by compiler). I think it's better to not allow this syntax because it leads to less readable code and potential bugs. Hence it's by default a warning on clang tidy. :)

[vfdev-5]

@datumbox I totally agree ! Definition like that DenseNetImpl x = 100; does not make much sense.

[fmassa]

Thanks a lot Vasilis!