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How to pass a vector to a stl function that takes const [] (C ++)

I have a 3d stl vector,

vector<vector<vector<double> > > mdata;

i also has the function

myfun(const double ya[]);

more precisely, it is a function from the GNU Scientific Library,

gsl_spline_init(gsl_spline * spline, const double xa[], const double ya[], size_t size);

but this is not related to my problem.

so now I want to pass the "last" data dimension to myfun. i tried this:

for (int s = 0; s < msize; s++) {
    accelerators = new gsl_interp_accel*[msize];
    splines = new gsl_spline*[msize];
    for (int i = 0; i < msize; i++) {
        accelerators[i] = gsl_interp_accel_alloc();
        splines[i] = gsl_spline_alloc(gsl_interp_cspline_periodic, msize+1);
        gsl_spline_init(splines[i], &(*mgrid.begin()), &(*mdata[s][i].begin()), msize+1);
    }
}

But the compiler (g ++, 64bit, Ubuntu) complains:

In member function std::vector<std::vector<std::vector<double, std::allocator<double> >, std::allocator<std::vector<double, std::allocator<double> > > >, std::allocator<std::vector<std::vector<double, std::allocator<double> >, std::allocator<std::vector<double, std::allocator<double> > > > > > SimpleAmfCalculator::interp_m(int)

: Calculator.cpp: 100: error: cannot convert ' std::vector<double, std::allocator<double> >*

to' const double*

for argument '3 to' int gsl_spline_init(gsl_spline*, const double*, const double*, size_t)

make: *** [Calculator.o] Error 1

Any help is greatly appreciated!

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4 answers


So the following works for me:



#include <vector>

void fun(const double data[])
{

}

int main()
{
    std::vector<std::vector<std::vector<double> > > data3d;
    ....
    fun(&(data3d[0][0].front()));
}
0


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You can pass the address of the first element, for example:

#include <vector>

void fun(const double data[])
{

}

int main()
{
    std::vector<std::vector<std::vector<double> > > data3d;
    ....
    fun(&data3d[0][0][0]);
}


Items vector

are kept contiguous. So this is the standard way I hope :)

23.2.4 Class template vector

1 Vector is a kind of sequence that supports random access iterators. In addition, it supports (amortized) persistent time insertion and end-of-end erase operations; insert and erase in the middle of linear time. Storage management is handled automatically, although hints may be for efficiency. elements of a vector are stored

adjacent , which means that if v

is a vector, where T is some type other than bool, then it obeys the identity:

&v[n] == &v[0] + n for
all 0 <= n < v.size().
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This screams for a general solution.

template<typename T, typename A>
T* PointerOf(std::vector<T,A> & vec)
{
    return &vec.at(0);
}

template<typename T, typename A>
const T* ConstPointerOf(const std::vector<T,A> & vec)
{
    return &vec.at(0);
}

myfun(ConstPointerOf(mdata[s][i]));

Edit: I added a template parameter for the vector allocator as suggested in the comments; I also used in () instead of [], so I wouldn't have to check for an empty vector, and I added a second version of the function for a const pointer.

+5


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I think,

&(*mdata[s][i].begin());

returns std:vector

type double

.

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