DAG.width needs to be updated to match QuantumCircuit definition

Information

• Qiskit Terra version: master
• Python version:
• Operating system:

What is the current behavior?

qr = QuantumRegister(5,'qr')
cr = ClassicalRegister(5, 'cr')
ghz = QuantumCircuit(qr, cr, name='ghz')

ghz.h(qr[2])
ghz.cx(qr[2], qr[1])
ghz.cx(qr[1], qr[0])
ghz.cx(qr[2], qr[3])
ghz.cx(qr[3], qr[4])
ghz.draw()

ghz_dag = circuit_to_dag(ghz)

print(ghz.width(), ghz_dag.width())

gives: (10, 5).

Steps to reproduce the problem

What is the expected behavior?

Suggested solutions

DAG width needs to count both qubits and cbits for self-consistency.

[jwoehr]

Per @ajavadia 's comments after merge of #2708 I’ll proceed to

1. Search for invocations or mentions of DAGCircuit.num_cbits() and change to DAGCircuit.num_clbits()

• qiskit-tutorials/qiskit/terra/advanced_circuits.ipynb
• qiskit-aqua/qiskit/aqua/utils/circuit_utils.py
• qiskit/docs/terra/custom_gates.rst

1. Remove DAGCircuit.num_cbits()
2. Search for current usage of DAGCircuit.width() and replace with DAGCircuit.num_qbits()
3. After such invocations are changed, DAGCircuit.width() can be changed to return the total number of bits (qubits + clbits) fulfilling the intent of this issue.

[nonhermitian]

It probably breaks several things which is why it has not been implemented yet.