One instance by construction

A protected constructor blocks outside construction. A static get() builds the single instance on first call and returns the same handle after.

protected beats local here. Subclasses stay possible for project-specific configs, while arbitrary construction stays blocked.

Singleton status rests on access control plus convention. A subclass constructor could still build a second object elsewhere, so the team treats get() as the only source.

Parse once, freeze before build

The test fills the config up front, then freezes it. Components read settings during build and run, and nobody writes after the freeze.

Plusargs face outward to regression scripts. The config object faces inward to components, and the test converts between them in one place.

$value$plusargs("N_TXNS=%d", n) reads a value given on the simulator command line as +N_TXNS=50 and returns 0 when it is absent, so the default stays in force. Plusargs and configuration covers it in full.

Global access has a cost

Any component can reach the singleton, so the dependency hides. Passing config handles down at construction keeps the same values with visible coupling.

One flat global also resists reuse. A block environment tied to it cannot run twice with different settings, which is the gap hierarchical config lookup later fills.

Testbench

The singleton machinery plus a testbench that proves shared identity, plusargs defaults and the freeze flag.

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class tb_config;
  protected static tb_config m_inst;
  protected function new();
  endfunction

  static function tb_config get();
    if (m_inst == null) m_inst = new();
    return m_inst;
  endfunction

  int unsigned n_txns = 100;
  bit enable_cov = 1;
  bit err_inject = 0;
  int unsigned timeout_ns = 100_000;
  string test_name = "smoke";
  bit frozen = 0;

  function void freeze();
    frozen = 1;
  endfunction

  function string convert2string();
    return $sformatf("test=%s n=%0d cov=%0b err=%0b to=%0dns frozen=%0b",
                     test_name, n_txns, enable_cov, err_inject, timeout_ns, frozen);
  endfunction
endclass

PASS: one shared config, plusargs defaults hold, freeze is visible

Related topics

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