error: default initialization of an object of const type 'const ext::function<Real (Real)>' (aka 'const function<double (double)>') without a user-provided default constructor

[ryandesign]

Hello, QuantLib 1.22 fails to build on OS X 10.11.6 using the compiler Apple LLVM version 8.0.0 (clang-800.0.42.1) that comes with Xcode 8.2.1:

nthorderderivativeop.cpp:59:41: error: default initialization of an object of const type 'const ext::function<Real (Real)>' (aka 'const function<double (double)>') without a user-provided default constructor
        const ext::function<Real(Real)> emptyFct;
                                        ^
                                                {}

Here's a full build log from the MacPorts buildbot system.

QuantLib claims to only require C++11, and Apple clang 500.2.75 and later are supposed to support C++11, and I am manually adding -std=c++11 to CXXFLAGS to turn C++11 mode on (since the build system doesn't do it automatically), so I would expect this to work.

Building on OS X 10.11.6 with open source clang 9.0.1 as installed by MacPorts works.

Building on macOS 10.12.6 with Apple LLVM version 9.0.0 (clang-900.0.39.2) from Xcode 9.2 works, as does Apple clang from newer Xcodes on newer macOS versions.

I'm not familiar with modern C++ so I wanted to report this in case maybe you are accidentally using C++ features that are from a later standard than C++11, or if this is a compiler bug then maybe there is something you could do to work around it.

[lballabio]

I'm a bit wary of saying it's a compiler problem, but it looks like it. May you try and see if this workaround fixes the issue?

diff --git a/ql/methods/finitedifferences/operators/nthorderderivativeop.cpp b/ql/methods/finitedifferences/operators/nthorderderivativeop.cpp
index b15eeeeba..a61e6e153 100644
--- a/ql/methods/finitedifferences/operators/nthorderderivativeop.cpp
+++ b/ql/methods/finitedifferences/operators/nthorderderivativeop.cpp
@@ -56,7 +56,7 @@ namespace QuantLib {
              "inconsistent number of points");
 
         Array xOffsets(nPoints);
-        const ext::function<Real(Real)> emptyFct;
+        ext::function<Real(Real)> emptyFct;
 
         for (FdmLinearOpIterator iter = layout->begin(); iter!=endIter; ++iter) {
             const auto ix = Integer(iter.coordinates()[direction]);

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[ryandesign]

May you try and see if this workaround fixes the issue?

That does get past that error. The next error I see is:

fdmtimedepdirichletboundary.cpp:33:34: error: constructor for 'QuantLib::FdmTimeDepDirichletBoundary' must explicitly initialize the const member
'valuesOnBoundary_'
    FdmTimeDepDirichletBoundary::FdmTimeDepDirichletBoundary(
                                 ^
../../../../ql/methods/finitedifferences/utilities/fdmtimedepdirichletboundary.hpp:63:54: note: 'valuesOnBoundary_' declared here
        const ext::function<Disposable<Array>(Real)> valuesOnBoundary_;
                                                     ^
fdmtimedepdirichletboundary.cpp:41:34: error: constructor for 'QuantLib::FdmTimeDepDirichletBoundary' must explicitly initialize the const member
'valueOnBoundary_'
    FdmTimeDepDirichletBoundary::FdmTimeDepDirichletBoundary(
                                 ^
../../../../ql/methods/finitedifferences/utilities/fdmtimedepdirichletboundary.hpp:62:42: note: 'valueOnBoundary_' declared here
        const ext::function<Real (Real)> valueOnBoundary_;
                                         ^
2 errors generated.

[lballabio]

Try removing the const from the two declarations listed (ql/methods/finitedifferences/utilities/fdmtimedepdirichletboundary.hpp, lines 62 and 63.)

[ryandesign]

The next error then is:

numericaldifferentiation.cpp:72:37: error: default initialization of an object of const type 'const ext::function<Real (Real)>' (aka 'const function<double (double)>') without a user-provided default constructor
    const ext::function<Real(Real)> f;
                                    ^
                                     {}
numericaldifferentiation.cpp:102:37: error: default initialization of an object of const type 'const ext::function<Real (Real)>' (aka 'const function<double (double)>') without a user-provided default constructor
    const ext::function<Real(Real)> f;
                                    ^
                                     {}
numericaldifferentiation.cpp:129:37: error: default initialization of an object of const type 'const ext::function<Real (Real)>' (aka 'const function<double (double)>') without a user-provided default constructor
    const ext::function<Real(Real)> f;
                                    ^
                                     {}
numericaldifferentiation.cpp:156:37: error: default initialization of an object of const type 'const ext::function<Real (Real)>' (aka 'const function<double (double)>') without a user-provided default constructor
    const ext::function<Real(Real)> f;
                                    ^
                                     {}
numericaldifferentiation.cpp:175:37: error: default initialization of an object of const type 'const ext::function<Real (Real)>' (aka 'const function<double (double)>') without a user-provided default constructor
    const ext::function<Real(Real)> f;
                                    ^
                                     {}
numericaldifferentiation.cpp:279:37: error: default initialization of an object of const type 'const ext::function<Real (Real)>' (aka 'const function<double (double)>') without a user-provided default constructor
    const ext::function<Real(Real)> f;
                                    ^
                                     {}
6 errors generated.

If I remove these consts in test-suite/numericaldifferentiation.cpp, then the build succeeds.

It sounds like the compiler is suggesting that inserting {} where indicated would fix this. If I leave the const and add the {} where shown, that build also succeeds.

[lballabio]

Thanks! Leaving the const and adding the {} is probably better. I'd try that with the previous errors, as well.

Once it works, may you create a pull request with the changes?