The flycheck-ycmd backend does not work in c++ mode

[ghost]

Description

I open ycmd layer, but in *.cc file, flycheck-ycmd does not work.
The complete works OK.
Run (SPC e s) ycmd does not work!

Add

(add-hook 'c-mode-hook 'ycmd-mode)

in init.el, the flycheck-ycmd works in c-mode, but cann't complete in c-mode.

Reproduction guide

• Start Emacs
• add ycmd layer, open a *.cc file.

System Info

• OS: darwin
• Emacs: 25.0.92.1
• Spacemacs: 0.105.16
• Spacemacs branch: develop (rev. 0ec385c)
• Graphic display: t
• Distribution: spacemacs
• Editing style: vim
• Completion: helm
• Layers:

(spacemacs-helm spacemacs-ivy erc emacs-lisp
                (c-c   :variables c-c  -default-mode-for-headers 'c  -mode)
                go html
                (latex :variables latex-enable-auto-fill t)
                lua markdown
                (rust :variables rust-enable-rustfmt-on-save t)
                shell-scripts sql yaml
                (git :variables git-magit-status-fullscreen t magit-push-always-verify nil magit-save-repository-buffers 'dontask magit-revert-buffers 'silent magit-refs-show-commit-count 'all magit-revision-show-gravatars nil)
                github
                (version-control :variables version-control-diff-tool 'git-gutter  version-control-global-margin t)
                (dash :variables dash-helm-dash-docset-path "~/Library/Application Support/Dash/DocSets")
                fasd
                (wakatime :variables wakatime-api-key "b0d4cb91-e437-470c-b54c-72d92716287c" wakatime-cli-path "/usr/local/bin/wakatime")
                (spacemacs-layouts :variables layouts-enable-autosave t layouts-autosave-delay 300)
                (ycmd :variables ycmd-server-command
                      '("python" "/Users/zwb/bin/ycmd/ycmd"))
                (auto-completion :variables auto-completion-enable-help-tooltip t auto-completion-enable-sort-by-usage t auto-completion-tab-key-behavior 'complete)
                better-defaults
                (chinese :variables chinese-enable-fcitx t chinese-enable-youdao-dict t)
                cscope gtags
                (ibuffer :variables ibuffer-group-buffers-by 'projects)
                (org :variables org-enable-github-support t)
                osx
                (shell :variables shell-default-shell 'multi-term shell-default-term-shell "/usr/local/bin/zsh")
                smex floobits
                (syntax-checking :variables syntax-checking-enable-tooltips t)
                ahei)

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