Remove global variables

[jngrad]

Global variables make code difficult to read and maintain. Tracking their origin is an annoyance, and name collision with local variables is always a risk. Should we consider prefixing global variables with g_, just like we prefix class member variables with m_ and enum values with their enum name? Or enclosing them in namespaces, like enum values from enum classes?

Here is one special example, hiding in the global scope of some LB code:

espresso/src/core/grid_based_algorithms/lbgpu.cpp

Line 115 in b25c014

int i;

Documentation is also affected by this situation, because Doxygen doesn't have a reliable scope resolution mechanism (Doxygen incorrectly states this variable is referenced by 380 functions).

[fweik]

The one you mention is also a linking hazard. In general most should also be static.

[jngrad]

Other example: there is a global variable energy:

espresso/src/core/energy.hpp

Line 38 in 182020a

extern Observable_stat energy, total_energy;

which can easily be shadowed in functions calculating energies, forcing the use of alternative variable names, like _energy, eng, ret.

Removing globals is also an option: e416e7e, d47a731, 34b8821, fab58ca, 6b56534

[jngrad]

Progress report:

• thermostat/integrator global variables were moved into structs
• global variables associated with the Mersenne Twister were removed (0abe004)
• many obsolete globals were removed from the Python interface (1cb64f2, 80a7907, 2ba1480)
• most accesses to local_cells have been removed (Reduce uses of the local_cells global #2899) and the last occurrences are now being accessed through a function (Expose local_particles only thru function calls #3501)

[jngrad]

Progress report:

• removed CUDA-related globals: a07a124 955ad2a 1d6c7ee fe8ddaa 436ce07 823b2f2 bb02c86 235480b fd3f7fd
• removed cellsystem-related globals: 6a3e364 4bab12f 9a11ffc 3ef73b7 84690b2 c829dd9
• removed electrostatics-related globals: 05f5a06, 54ea3b5
• removed other globals: c4a3103 ba0bf77 4b4e774 4923410
• combined int fluidstep and double fluidstep into a single global: 37e7e4c